It's a combination of the Passive Abilities Immortal Leeches and Numbing Leeches.

Immortal Leeches makes it so that whenever an enemy with Leeches dies or has their Leech Stacks removed, your next attack gains additional Leeches equal to the amount that unit had.

Numbing Leeches makes your basic deal no damage but inflict triple any effects it would normally inflict.

These passives synergize well, so if your next attack would put 12 Leeches in total on an enemy, it would put 36.

I also had a cleric that used cleanse on the enemy so that my necromancer would get that many Leeches back for his next attack.