Elkir, the Necromancer is a very summon based AP champion. His spirit walkers actually reduce his maximum health while active, but his potential damage and durability is quite high if he is able to maintain and micro a large number of summoned creatures.
His passive is used to power all of his other abilities. Not sure if this is good or not, but I like the idea of him having to kill stuff before reanimating it.
Spirit walkers are sort of tanky but won't automatically aggro champions until he gets his ultimate and manually controls one. Their unique ability is an aoe slow + damage which is designed to help Elkir keep people off of him.
Ghasts are squishy but they go fast, attack fast, and do a bunch of damage. Their ability makes them do more damage.
Soul Rip is a skillshot that does minor damage and then drains health based on the number of souls you have. It's your sustain tool and can do a lot of damage if someone stays near you for its full duration. Given that if he has 4 Spirit Walkers out at once, his maximum health will only be 40% of its normal value, I decided that he needs a really powerful lifedrain.
His ult gives him control of one creature. It doesn't have a cooldown. Basically, you are supposed to use it to micro your creatures really hard. It also does some other stuff but that doesn't matter.