The DM doesn't wish for anything. The NPC does.
This attitude is leading everyone down the wrong path. You're thinking "If I were playing a PC who was an evil cultist, what would I wish for?" But what really makes this wish unique is that it's a
DM's wish. A DM doesn't need to wish for a half-fiend, some demons, or a horrid wilting. A DM gets that stuff for free to build encounters with whenever he wants. What we want is to use wish to do some things DM's normally can't do, so that this will be special.
The "turn the ground to lava" one is a good start. You can make an encounter over lava, but just all of a sudden the forest is gone and lava is there? That's using
wish to freak out the PCs and take them off guard. That might be a 12th-level spell it's duplicating, though, seems too good.
I would like for the cultist to wish for his enemies to believe in the cult as he does. Kind of duplicating dominate person, but with some real mind control so they don't remember anything different than worshiping glabrezu.
How about "I wish the glabrezu would fight them himself?" That lets you set up the interesting encounter of a demon who doesn't want to fight and is fighting against his will. It might turn the encounter really funny to see how annoyed he gets with that puny cultist.
In the subverted wish category, you could have him wish "I wish I were at the portal-opening ceremony instead of here," and he disappears, only to be found later being used as a material component for said ceremony.
Or have him wish "I wish I were strong enough to fight these guys all on my own!" and turn into some bizarre creature, in an unpleasant metamorphosing process.