yeah. it eats 66 hp per second while giving you large stat bonuses (in my case 160 to attack, 33 to def and 66 to everything else). there's a good way to minimize it though. enable it, use glyph union and immediately disable.
also, was anyone able to find any use for the back cerberus glyph (one that makes a wolf head hover over you) other than to open the gate? it just doesn't seem to do anything.
edit: nevermind. i just noticed that it adds some attack bonus the more you're hurt. on below 20 hp with a lance equipped i had like +160 attack.
for a fun time, i equipped two death rings, melio falcis, nitesco, arma custos and full minerva armor, then lowered hp to 1. after activating arma custos it made me deal 2000+ per glyph union hit to drac and killed him within 6 seconds before he had a chance to finish his first move.
new game+ only though, since there's only 1 death ring in the game.