Necromancer

General Mulligans

Undertaker can get out of hand really quickly if not removed, with so many cheap deathrattle minions in the deck. If possible, you ideally want your raise dead to give you an undertaker or two to be used again on a later turn with cards like twilights call.
Grandmummy is especially strong when paired with undertaker, as it will trigger undertaker's effect twice.
Leper gnome is simply a cheap option on turn 1 if you don't mulligan undertaker, but can also be used in combination with reliquary of souls or another leper gnome on the following turn to trigger undertaker twice.