I just tested it on a dummy, the description reads that it absorbs 100% of damage dealt as hp, but when I used it, it consistently only drained 44 hp, even though it was doing around 400-500 damage.
Edit: nevermind, figured it out, it will only absorb as much hp as the monster has left, so even if you do 1000 damage if the monster only has 44 hp left you will only absorb 44 hp.