Minecraft 1.20.5 has changed syntax on many commands, so most commands will no longer work without an update.
An update is in progress, you can check compatibility progress here.
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Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft Wither NBT Data Tags
Using the summon command to spawn in a minecraft wither is simple, take the command and enter into chat and presto you have a wither.
How to summon a Wither with invulnerability.
Withers have an invulnerability timer when summoned, this counts down at 20 ticks per second.
/summon minecraft:wither ~ ~ ~ {Invul:1200} (Invulnerability for 60 seconds)
/summon minecraft:wither ~ ~ ~ {Invul:6000} (Invulnerability for 5 minutes)
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If you're looking for a particular summon mob generator you can link to these specific versions of the command generator.