Summoning wither skeletons

[3Divi]

The tablet of necromancy allows to summon mobs, is it possible to summon wither skeletons?

[CanVox]

Not sure.  The only ways I know of to get ahold of wither skeletons are:

• Wither infusion ritual
• Gaia guardian comes with a few, so safari net one & use a spawner
• Two wither skulls are in the floating city

[swat_raptor]

I've tried a dozen or so ways of getting a wither skeleton to spawn and I'm 99% sure that the gaia guardian is the only way available,