Combat profile
For Minecraft 26.3 / NeoForge, wave base values and supported behavior can be changed per registry entry: see health, damage and base armor and mob abilities. Tier, size and wave scaling still apply.
Behavior
It retains vanilla Wither flight, immunity and side-head attacks, but living targets cannot pull its navigation away from the Nexus. When idle it closes to roughly 12 blocks and fires IM Wither Skulls that damage the Nexus.
A Wither created by merging
Four same-Nexus IM Wither Skeletons can gather and merge. The resulting Wither retains their Nexus and starts with current and maximum health equal to twice their summed remaining HP, at least 1 HP, instead of the normal spawned-Wither health formula above. Before wave 25 recruitment is limited to ten blocks from the leader; at wave 25 and later it includes distant loaded same-Nexus skeletons. They must still gather within the 3.5-block merge distance.
Drops
Uses the vanilla Wither loot table, including its Nether Star drop.
Vanilla replacement
During an active Nexus invasion, a vanilla Wither may be converted to its IM counterpart with 30% of its health.