How do I make a drain or leech spell that steals HP?
The Easy Way[edit]
To make a spell that steals all damage done, just enable the Absorb Damage bitset in the Attack Editor.
The Hard Way[edit]
If you want to make an attack that only steals PART of the damage, the process is longer. Say you want an attack that only absorbs half of the damage done. First, create an attack to do whatever damage you want and chain it to a second attack. The first attack should have a Do Not Chain if Attack Fails bitset.
Leave an empty image for the second attack or give it a healing appearance. Give the second attack a target of "Self". Assign it Pure Damage math, an Attack stat of "previous attack", Never Misses accuracy, and -50% Extra Damage. Turn the Cure Instead of Harm, Do Not Randomize, and Ignore Attacker's Extra Hits bitsets on.
Thus, after someone uses the first attack, he/she/it should gain half of the damage dealt.