In the campaign I'm playing in, there is internal conflict in the upper planes, and one of the lead instigators is a solar. We've killed it multiple times, but it keeps coming back with reinforcements. The party plans to go to the upper planes to finish it once and for all... But the DM says that it'll be stronger in its native plane.
Is this typically true – are outsiders stronger in their native planes? Is this true for other outsiders?
I'm fine if it's homebrew, but I'm just curious.
There is nothing in the Monster Manual that would indicate so, if you refer to the solar's stat block. They do not even have lair actions.
Maybe what your GM means is that in his home plane he will have access to a much larger pool of resources, commanding large numbers of lower-ranked angels and celestial creatures, and will have access to his fortresses defense measures and maybe directly can call for or invoke his patron deity to come to his aid.
Or your GM's going to homebrew some lair actions and other power boosts. Wouldn't seem out of range. It's commonly accepted that the high-CR opponents are too weak to challenge a high-level party anyways in 5e.
