This troll is no joke. He's entirely invisible, blinks around, spawns adds, and does a lot of melee damage. Good luck!Though you can't see him directly, you can see the splashes he makes every time he moves thorough liquid, and he still shows the hit / miss /
block animations if attacks hit his square. Also, the troll foals he spawns are visible, and when one spawns, you can be sure he is on an adjacent square. Grenades and shotguns are recommended.
Since the jump skill does not allow you to jump to an occupied square, if he hits you in melee, try aiming the jump skill around you. If one of the spaces adjacent to you flashes red in spite of being empty, then he is on that square. Similarly, the Light Manipulation mutation's targeting line will show a red "blocked" square if it is aimed through him, so you can use it to reliably zero in on his position. NOTE: This trick has been targeted in a recent bugfix and might not work anymore. You can alternatively use the forced attack command to feel out his location if he is already in melee range, since trying to attack unoccupied tiles does not use up turns.
If you are playing a melee-oriented, axe-using character who has Hook and Drag and you are able to work out which square he's in when he is adjacent to you, hit him with Hook and Drag. If you successfully grab him, he'll be locked down in that same square until he can break free, letting you hack at him repeatedly in a satisfying fit of vengeful frustration. This is a superb opportunity to activate Berserk and show Haggabah your best Jotun impression.
The Kick Back skill can also be effective as he will continually try and reenter the square you kicked him out of, letting you make short work of him if you have an automatic weapon or flamethrower.
Oddly, it seems you can contain the troll within a circle of force walls. It makes no attempt to teleport out, and cannot grow foal spawns (as there's no adjacent tile). Conversely, you can attempt to manipulate where they walk by making a line of force walls instead.
Haggabah is found in the Bethesda Susa healing pools.
Haggabah can spawn Troll Foal.
He drops the Gold Key for Bethesda Susa.