Hardcore Wither

 The more Withers you fight, the harder the fight gets, and the sweeter the rewards.

Withers spawned around you get increasingly difficult special abilities. If a Wither is spawned with multiple players around, all of their experience with Withers is taken into account.

There are several special abilities (boosted health) most of which are intended to be discovered the hard way. All abilities can be disabled/adjusted in configs. Each ability also can be configured to be Blacklisted from use on a per-Dimension basis if it does play well there.  

There is also a way to get Tinkers' Construct Green Heart Canisters if that mod is present (disablable in configs).   Allows Nether Stars to be affected by the Looting enchant. (at a configurable rate).