Lilith

Lilith is a boss added by Witchery. To summon her, complete the Creatures of the Night ritual to summon Elle. When Elle is led to a large Lava lake in The Nether she will transform into Lilith, which has a total of health points. However, she regenerates health points every second. Lilith will fire fireballs at the player, when they are deflected back at her, she stops healing for a while. When defeated Lilith shrinks and can be given:
 * a Glass Goblet to fill with her blood, allowing the player to become a vampire.
 * a Poppy to promote a level 6 vampire player to level 7.
 * a tool, weapon, or armor piece to bestow a random enchantment on it.
 * a piece of Garlic to cure the player of their vampirism.

Once one of the aforementioned items are given to Lilith she disappears afterward.

Lilith is capable of casting magic of her own like sending the player flying away, pulling the player towards her, and giving them slowness. She shoots fireballs that take away health points. Another attack she has is raining fire upon the player from the sky.

Since Lilith has extremely powerful regeneration it is best to knock back her fireballs at her to momentarily stop her regeneration. You should also brew a brew of Demonbane and give it the gas effect to do a lot of damage to Lilith.