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2 basic builds are provided, one for aggressive players, and a defensive 'training wheels' option. Select team 1 / 2 to see the difference. (Cassiopeia icons above the table of contents and the mobafire advertisement)

The following chapters are under construction:
Combat and Matchups (needs update)
Abilities and Skilling Order (abilities explanations need rewrite)

Take a combination of Ignite, Flash, Ghost depending on your opponents.

Flash can work offensively as a gap closer, or repositioning tool or defensively vs AoE ultis Katarina, Kennen, Pantheon

I prefer Flash defensively in combination with Ghost for mainly non-healers with movement/zoning advantages Blitzcrank, Nunu & Willump, Anivia, LeBlanc, Katarina

I will (sometimes) take Exhaust if facing 3 or more melee dps Jax, Jarvan IV

The most effective combination for cassiopeia is Flash and Ignite

Take Teleport instead of Ghost if you would like to be more flexible or haven't mastered the basics. You can also us it in a hybrid support role, with Shurelya's Battlesong and Will of the Ancients

Core Items:
Doran's Ring early game synergy with Ignite mastery, good MP5, 100 HP
Giant's Belt early-mid game survivability item, built into Rylai's later
Hextech Revolver / Will of the Ancients 25% - 20% spellvamp (recently buffed from 20%-15%)
Rabadon's Deathcap Best AP item, makes Will of the Ancients and Deathfire Grasp multiplicatively stronger
Rylai's Crystal Scepter 500 hp & 15% slow on Twin Fang & Noxious Blast

5th/6th items:

Void Staff Grab this and ditch Sorcerer's Shoes for Boots of Swiftness
if team lacks tankline and/or damage output vs the opposite opponent team: build Boots of Swiftness, Giant's Belt, Hextech Revolver, Rabadon's Deathcap, Void Staff, Banshee's Veil, Rylai's Crystal Scepter, Will of the Ancients

Abyssal Mask Also replaces Sorcerer's Shoes, preferable finisher or against 2+ AP champs snd/or 4+ champs with 30-50 magic resistance champs, replaces Banshee's Veil

then choose from:
Negatron Cloak / Banshee's Veil For lack in MR. Negatron Cloak provides good synergy with sustained damage skirmishes and Banshee's Veil provides protection against dangerous ultis ( Warwick), protection for late game ganks, potential for powerful burst damage Flash + Petrifying Gaze + combo. Ideally Negatron Cloak is the last piece of mid/late game survivability to get, and good to get as soon as your team is dealing enough damage in teamfights (otherwise stick to AP)
Deathfire Grasp Anti-Carry Nuke (combo initiator) deals 50% of target HP at 570 AP with 30 flat or 40-50% penetration
Zhonya's Hourglass Situational, good vs burst, aoe, blitzkrieg teamfights. Very defensive


Defensive build for beginners:
The focus of this build is to balance Damage Output and Survivability.
Characteristics
Magic Penetration/Reduction > 8 reduction 15% penetration at lvl 1 (28 reduction with Haunting Guise
First Items > , , Meki Pendant,


To cheapen costs while maintaining presence and survivability in teamfights and lanes:
Haunting Guise replaces Sorcerer's Shoes
Meki Pendant replaces Doran's Ring
philosopher's stone provides lane survivability
Boots of Swiftness provide survivability and utility
with Giant's Belt next, the focus switches to AP, concluding with Deathfire Grasp.
Void Staff replaces Haunting Guise after Rylai's Crystal Scepter and Hextech Revolver are complete.

sell philosopher's stone soon after / when purchasing Hextech Revolver
upgrade to Shurelya's Battlesong for more defensive play (sell for Banshee's Veil)

x2 = +3% movement speed

Combined with the Quickness Mastery, 6% additional movespeed grants 22 movespeed with Boots of Swiftness or 23 movespeed with Sorcerer's Shoes

8.1% cooldown at level 18

17% cooldown total, without Deathfire Grasp.

Greater Glyph of Ability Power is also an option

24 magic resistance

For lack in magic resistance. (feel free to choose seals that fit your playstyle)

Other options are: Greater Seal of Scaling Cooldown Reduction, Greater Seal of Health, greater seal of vitality, Greater Seal of Ability Power, Greater Seal of Scaling Ability Power.

8.6 magic penetration

Helps killing minions and dealing some extra damage. Runes bring magic penetration to 10.5

1.9 magic penetration

Offensive rune, with 20 penetration purchased in-game, a squishy's MR will be -0.5.

Greater Quintessence of Movement Speed, Greater Quintessence of Ability Power, Greater Quintessence of Magic Resist are also options

Stay out of melee range.

Giant's Belt / Rylai's Crystal Scepter will let you take some (magic) damage. Your 20% - 25% spellvamp will keep you charged up in extended teamfights. Cassiopeia does not excel in fighting in melee range, but she can fight melees with proper micro.

You will need to reposition yourself in between every couple of attacks when vs a melee. Speed, flash, and your ulti will be most effective vs casters, ranged, and skillshots.

Melee champions with haste/dash abilities (Yi/Tryndamere/Shen) are very difficult matchups for Cassiopeia, and often will challenge you under your towers (you will need to land a frontal Petrifying Gaze to consider your towers a safe zone vs a Tryndamere type).

Evasive and Disabler mages, support, and ranged DPS are also tricky to approach, such as Morgana, Twitch, Teemo, Soraka?, Fiddlesticks, Zilean, Kayle?

Tanky DPS's or Bruiser with disables, with movement abilities are rarely threatened by Cassiopeia when played well, such as Jarvan IV, Renekton, Tryndamere, Trundle, Udyr


TL DR: Avoid heavy hits from melees, and heavy counters to your slow/haste advantage


Cassiopeia is no doubt a great zoner ( Kog'Maw, Urgot, Anivia are better ofc), so with all her movement CC, bonuses, and items I see her as a character that benefits mostly from good micro and positioning (also why she is a tough nut to crack) ... as well as an easy farmer

an example of a quick skirmish vs Teemo:

see teemo too close
position
Noxious Blast to initiate

second 0
Noxious Blast hits teemo and poisons
move in
Twin Fang twice to damage
reposition to cover teemo escape and land future E + E + Q
or reposition to be able to back off easily
>> (during 0-3 sec) speed buff 15% - 25%, 75-235 damage mini nuke

second 3
Miasma to slow future movement
Noxious Blast to regain speed buff
reposition

Petrifying Gaze
Twin Fang twice to damage
Ignite before stun wears off

Flash

base damage @ lvl 6 (with mastery) - damage with ignite - & dorans ring - with dorans ring and ignite - (% increase) - damage vs 30 resist (77%) - damage with 10 penetration (83%) - 20 pen (91%)
Noxious Blast = 115 / 118.24 / 127.24 / 140.74 (20% increase) / 108.4 / 116.8 / 128
10 penetration = 8.5 damage,
780 magic damage in about 5 seconds, with a 2 second stun following @ level 6

it won't hurt to first soften up teemo with a couple of Miasma hits, it will also build stacks on Deadly Cadence

Abilities

- Unique Passive

helps spam spells to create combos and keep up combos, by giving Cassiopeia's abilities diminishing mana costs (50% on 5 stacks)

- Q

early game - use to harass and poison opponents who get too close to you. Punish those who stand still or have limited movement possibilities.

You can use it to check bushes (watch for your speed buff) or stealths

Use liberally to farm. Use to farm minions in groups of 3

generally - use it to prep (passive stack + poision) and initiate (speed buff) your attacks. Poison and move buff last 3 seconds, after which you should have placed a Miasma to slow and be able to land another Noxious Blast to then reposition for another couple Twin Fang nukes.

- W

early game - Use whenever it is mana efficient to harass (non-tanks, non-lifestealers/healers). Use to push minions faster (land to poison an entire creep wave as it moves). Use for slow to finish kills.

mid to late game - use to slow, escape. Do not use to for its damage. Be wary of its mana cost

generally - if u can land a W to pull of a kill on an out of position opponent use it, otherwise keep it off cooldown after laning phase. It has the longest cooldown, highest cost, least damage output, and worst ap ratio.

- E

Your bread and butter spell. Be careful not to use it on an non-poison targets. Singed makes for a powerful team combo with Twin Fang

Petrifying Gaze - Ultimate

long cooldown, hard to place & land stun, low damage at ranks 1 & 2, necessary to keep on cooldown as a defensive component, good for scoring kills, 1 v 2 fights, tower defense, saves teammates in a teamfights. Use it with good judgement.

Best Uses:
Stun charging melee squishy: Miasma, Twin Fang, Noxious Blast, Twin Fang, Twin Fang Ignite, chase
In 5v5 fights: stun 2+ high damage dealers, save 1+ allies, proceed to combo.

Since you are likely spamming spells, remember it has a 120-200 mana cost.


Skilling Order
level 1 Noxious Blast Early laning game bread and butter ability
level 2 Miasma controls your safe zone, good for combos, high cost. (recommended at lvl 1 for beginners)
level 3 Twin Fang use only for kills, lethal damage, and farming during downtime (only with spare mana).
Level both Twin Fang (E) and Noxious Blast (Q) next, as the lane requires
level 6 Petrifying Gaze Powerful with good timing. Opponents will typically underestimate their/your safe zone at lvl 6.


Level 7 Noxious Blast scales best at rank 3. Farm and harass. 55 mana, 3s CD: 155 AOE DoT, 20% haste on champ hit
8, 9, 10 Twin Fang ...time for another kill?
Level 11 Petrifying Gaze Ulti now deals 325 damage on a 120s cooldown, so not a significant upgrade

12, 13, 14 Miasma - You won't have your Rylai's, but your AOE slow will now double from 15% to 30%
Level 15 Noxious Blast
Level 16 Petrifying Gaze now deals 450 + 60% ap, so it will be a nice squishy killer regardless of whether they get stunned for 2 seconds or slowed by 60%. Be sure to always have 200 mana for it, and use it wisely.

Level 17 Noxious Blast
Level 18 Miasma

Shurelya's Reverie 1400 gold (upgrade cost)
Shurelya's Battlesong
Defensive item. Helpful for positioning, ganking, escaping, chasing, kiting.
Includes CD reduction, Health, HP5, MP5

This is the only non-AP item in this build, and is the main reason why this build provides only 380 AP with all items (1000 mana from Tear of the Goddess). Replacing this with a Rabadon's Deathcap gives an additional 315 AP, a good endgame option

This item will expand your expand your safe zone in teamfight, allow your team to catch fleeing enemies, allow for faster ganks, protects you against your worst enemies (Yi, Tryndamere, Teemo, Eve, Shaco, Twitch etc), and allow you to farm/push solo safely (Cassiopeia is an excellent farmer).

Use it often, and use it safely.

sell this item for Will of the Ancients for 560 gold for late game survivability and health regeneration

Your Shurelya's shouldn't be inactive for more than a minute or so, use it to farm if there is nothing better to do. You will need lots of farm for your next item ( Rylai's Crystal Scepter) to begin snowballing.

Rylai's Crystal Scepter 2245 gold
Rylai's Crystal Scepter
+80 AP, +70 HP, Twin Fang (E) now slows your target by 15%(!!), changing opponents' commitment threshold.

This is an important purchase, and you should try to catch your opponents off guard with it (e.g. little teemos nom nom) (twin fang damage - 60/95/130/165/200 +55% ap). Do not upgrade Giant's Belt to Rylai's Crystal Scepter before


Hextech Revolver 1200 gold
Hextech Revolver
Your opponents are playing smarter against you or they have fed you to no end (You now have 2 hastes, 1 AOE slow, flash/ignite/ghost, and 1 slowing nuke). Whether they keep their distance and stay behind the tank line, or create smaller teamfights, or have a standoff in mid, - Spellvamp will increase your durability for longer skirmishes, and reward you for farming after a battle.

Archangel's Staff 1860 gold
Archangel's Staff
Not a lot of flavor but, it will come at a good time, giving you an additional 18 mana per second and a whopping 150 AP for 1860 gold (3550 mana at level 18).

Will of the Ancients 900 gold
Will of the Ancients
You will start to hit less hard as your opponents build more durability. You will be exponentially more potent with tankier teammates, so get Will of Ancients ASAP, and try to encourage your team to prolong teamfights somewhat (or focus all together tanky teams) and you should end up with a nice ace.

Zhonya's Hourglass3300 gold
Zhonya's Hourglass
I rarely get to use this. Typically I have spent a 500-1500 gold on wards and the item is expensive and slow to build (and wards take up a slot). Be careful in deciding whether to purchase the Needlessly Large Rod or the Armor first depending on the opponent team. Buying armor first means you will need 2600 gold before completing the item and will probably be getting some elixirs in the meantime.

Teleport

teleport offers many advantages in synergy with items and abilities Shurelya's Battlesong, zoning, teamfight ulti and abilities, and sight wards

I like to orient Cassiopeia towards a strong early and mid game, since she becomes less and less useful in the late game. Teleport helps relane faster, have flexible shopping, and makes up for weaknesses in my team in defense and offense (tower-wise).

Also for a semi-backdoor, teleporting to a large friendly minion wave, placing two strategic sight wards, spamming Q and E to farm easily nets a tower or a chunk of tower hp. Once a gank approaches, Shurelya's Battlesong, Boots of Swiftness, Petrifying Gaze, Teleport, Flash are all tools to use to return safely or setup a counter-gank for my team.

I plan to add a few more sections, topics include:

warding / warding vs twitch/eve
laning
5v5 tower defense and pushing
ganking

these statistics need to be updated based on new runes, items, and masteries

most statistics listed by mobafire are incorrect.
at level 18 with all items purchased:

39% cooldown reduction
20% spellvamp
XXX ability power (695 AP when replacing Shurelya's Battlesong with Rabadon's Deathcap)
15% magic penetration
10.5 magic penetration (flat)

1730 hp (base)
830 hp (items)
43.85 hp per 5

XXX mana
XXX mana per 5 (base - 14.5)

133 armor
70 magic resistance (95 with Mercury's Treads)

422 base movement speed (with Boots of Swiftness [310 + 90] * 1.06)
403 base movement speed (with Mercury's Treads [310 + 70] * 1.06)
note: 80% of movespeed over 415 is given in-game

these are pro/cons regarding this particular build

Thoughts

Perseverance Mastery seems to lack in function, with all items, it increases MP5 by 1.6 and HP5 by 1.6. Surprisingly, Good Hands mastery is more useful, decreasing time spent dead up to 10%.

Build 1 When banshee's veil is useful vs. 3+ AP, and sturdy laners
Boots, Sapphire Crystal, Ruby Crystal, catalyst the protector, Sorcerer's Shoes, Negatron Cloak, Giant's Belt, Needlessly Large Rod, Rabadon's Deathcap, Banshee's Veil, Rylai's Crystal Scepter, Void Staff, Boots of Swiftness, Zhonya's Hourglass or Deathfire Grasp