I should note that I made this build for fun. It's silly, terribly trolly, and fairly easy to counter. You shouldn't use this in ranked, or you will probably get reported. I mostly made it because I was looking through my champion list and thought, "I'll bet that Cassiopeia could jungle with all her AoE damage". That being said, it is quite a bit of fun to actually use this build, so enjoy!

The biggest problem you have as jungle Cass is mana. You need a spirit stone as soon as you can get it. Once you get a chalice, you should be able to start donating your blue buffs to your mid laner (or, better yet, encourage your teammate to play a manaless mid laner like Akali, Yasuo, or Katarina). Liandry's torment and Rylai's are your two most important items. This is where most of your damage comes from.

The final build listed here is good for your average game. However, if the enemy team has a high amount of physical damage, avoid the abyssal scepter (in favor of a Randuin's); avoid the Athene's in favor of a Morellonomicon. This also means that you will build a Forbidden Idol over the Chalice early. You can grab an earlier Zhonya's hourglass if you are being bursted or focused a lot. Dropping your W directly onto yourself, and using Zhonya's can be a great way to deal with a melee attacker.

You can upgrade your yellow trinket either way, if you see fit. Red trinket can also be an option.

Building a deathcap isn't the best investment because Cassiopeia's AP scalings are far from stellar.

Your runes are entirely designed around letting you finish that first clear safely.

AP blues will improve your clear drastically, and no other type of rune fits well into that spot. If you feel you need it, you could go magic resist, but this will hurt you pretty badly in the beginning.

The health regen yellows are by far the most efficient when jungling. Since you are almost always at less than full health, they are always in effect, and will yield you much more effective health than armor or flat health.

Attack speed reds and quints let you make maximum use of that hunter's machete heal and bonus damage, which is what really gets you through the jungle the first time around.

Masteries are 9/21/0, once again to help you survive. Most of the mastery page is fairly self-explanatory. There are two things to note here. First, you take attack speed and attack damage in the offense tree. The attack speed is, once again, to make good use of that machete. The attack damage is much more efficient in the first clear than the AP, so take it since the missing 22 AP won't really matter towards the end of the game. Second, the Oppression mastery will essentially reduce your damage taken in the jungle by 3% because of the slow on your W. This is absolutely essential.

Your summoners are pretty simple. As a mage without a gap closer, flash is 100% necessary. Smite is necessary for jungling. It's really that simple.

It seems like you should be using your E to farm as fast as possible. However, using your E regularly in the jungle will run you out of mana very quickly for relatively little damage return. Instead, use your W and Q. You will clear the group camps very quickly (wight is tough for Cass, just like a lot of other AoE junglers). You will max your W first for two reasons:

1. Upgrades increase the slow, which will increase your resilience to counterjungling, as well as make your ganks slightly better.

2. All of Cassiopeia's skills cost an extra 10 mana per upgrade. You can't really afford to use your E a lot in the first place, so you don't want to upgrade that. The damage increase per second per upgrade is roughly the same for Q and for W. However, W is on a 9 second cooldown, while Q is on a 3 second cooldown, making W by far the more mana efficient choice.

You will have to make good use of Cassiopeia's passive to sustain in the jungle. Between every camp, cast your q every few seconds to keep 5 stacks of your passive up. This will cut your mana costs in half, more than making up for the wasted q's you will be dropping in empty jungle.

You will max your E second because it is your main damage source against champions. By the time you are putting your 3rd or 4th point in this skill, you should be moving into the teamfight stage, meaning this damage output is very important. Furthermore, because you use your Q to keep your passive up when jungling, you will want to max this one last to make sure you don't run out of mana.

Cassiopeia's ganking abilities are lackluster to say the least. She is fairly vulnerable to counterganks, has no gap closer, and has no hard cc until 6. Generally speaking, unless you see a lane very out of position, you won't be able to gank. Just keep farming until you see an opportunity.

When you do gank, try to save your slow pool until the laner uses any gap closers they have. Start using Q and E (and autoattacks if you have red buff) as soon as you are in range. Do NOT use your E on someone unless they are either poisoned, about to escape (under tower or into their jungle), or about to die. You don't want to have that ability on cooldown in the middle of a gank.

Be extremely careful when ganking burst mages or other high damage champs (Le Blanc, Veigar, Zed, post-6 Jax, Lee Sin, etc.), as you are somewhat squishy.