Hey, played Cass in free week. Just some casual bot games. Thought she was kind of fun. Might buy her eventually.

Not in any order:

Two of these should solve early-game mana problems. Sell later on, and MP5/level glyphs should be plenty later.

I don't see this item discussed often in connection with Cassiopeia, but I don't know why. It seems really good, unless the details of common gameplay experience prove the first impressions to be misleading. Cassiopeia has nigh-nonexistent cooldown times and insane AP scaling, which makes this a pretty obvious candidate in my mind. Maybe this is put to better use AFTER deathcap, but I like having the MRes before too long.

Might as well stay alive. Skip it if you feel cocky, I guess, there's always Archangel's Staff. Maybe this can be postponed if you hold on to your Doran's Rings for a while.

No introduction needed, I hope.

Nice filler item for a one-stat champ like Cassiopeia. Lets you take Sorcerer's Shoes.

Might as well get some armor. Fairly situational. Get early or not at all as needed.

Masteries:

As much as I love 9/0/21, Cassiopeia is pretty uniquely non-dependent on CDR, making 9/21/0 my choice here. You can skimp on resistances and health some, or you can just be extra sturdy. I find that Lich Bane + defense masteries + maybe runes = enough MRes. Either skip on armor beyond runes/masteries/innate, or pick up an hourglass as needed.

Runes:

Glyphs/quints are iffy. If you don't need MP5, then MRes/level and MPen is nice. If you need MP5, then MRes/level and MP5 or vice-versa is good.

Spells:

Exhaust helps a lot with combos and zoning; those little delays can make it tricky to land them sometimes. Ignite is a natural aid to DoT. Others like Flash are good too, but these first two seem most obvious.