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Guardian golem can be used to fight Planar Sphere invaders
If you talk to the guardian golem, while owning the mage stronghold you can instruct the golem to ''follow me''. If you then lead the golem to the sphere entrance, the golem can also be instructed to ''stay here''.
This is very useful as the golem will attack any hostile people or creatures inside of the sphere, that means when the mage haters/assassins teleport in, the golem will automatically attack the invaders and the invaders will attack the golem.
The golem is also invincible and cannot be killed. This is great because 1) a golem can join you in combat which is pretty cool and 2) if you're playing a weak/cowardly character you can hide and let your golem fight for you.
I recorded a clip of the Golem fighting, and I've attached a YouTube link in this post: