Wither

The Wither is an undead, floating boss mob added by vanilla Minecraft. Unlike other enemies, the Wither can be summoned anywhere that blocks can be placed, including in other dimensions.

Advent of Ascension does not alter the Wither fight. However, it does add extra rewards for defeating it. The rewards are displayed below. Full information on this boss is available on the Minecraft Wiki, although some information on there may have changed between the Minecraft version Advent of Ascension is currently on and the latest Minecraft version.

Wither Loot Table
When the Wither is killed, it will drop items from the below loot table in addition to its other drops.

Abyss Realmstone
Summoning the Wither in the Nether with a Blank Realmstone in the inventory will convert the realmstone into an Abyss Realmstone, required to enter the Abyss. The Wither only needs to be summoned, not defeated, for this to occur.