The Corpsebloom is a Lunar item introduced in Risk of Rain 2.
Like all Lunar items, the Corpsebloom has a powerful effect and a drawback. It increases all healing by 100%, but limits how much the user can heal at once to 10% max health. Each bloom increases this amount by 100%, and reduces the maximum healing per second by 50%.
Healing is "stored", so if the user is healed to maximum life, and then take damage, healing received earlier may still heal them.