The Necromancer's first ability is Summon Skeleton. It will summon one Skeleton Card into play every turn.
Its second ability is Enrage. It has a chance to increase the attack damage of one of your Undead Creatures by 1 for this turn.
Its third ability is Heal. It heals one of your Undead creatures for 1, but only if you have Graveyard in play.
How to get
You can obtain this Card from the Base and Base Light Booster and the Empty Graves Deck.
The Necromancer is especially powerful with a Graveyard Card, as the Skeletons he summons also benefit from it. Equip him with a sturdy Armor to increase the time he can summon skeletons, or a Necromancer's Cloak to further support your Undead creatures.