Making my big bruiser more than a sack of HP and multiattacks

[DragonBaneDM]

Hey guys!

So I'm working on a double dragon type fight for my level 5 players, featuring two CR 3 monsters for a medium encounter. The first of which is a warlock themed caster that manipulates portals to summon in grasping tentacles, blasts of psychic energy, and teleport enemies to disadvantageous positions.

The second is his brother: a twisted abomination subjected to experiments by his brother. The former human is Large in size, and I know he's gonna have broken/twisted legs but huge arms. I'm having trouble coming up with ideas for his role in the encounter besides "choose between making a double attack or using your rechargeable ground pound AOE attack".

Any cool ideas for what an eldritch themed mutant could do during a fight to compliment/be manipulated into hindering his portal themed brother?

Ideas I've had include tricking the mutant into smashing portals, getting him teleported to the middle of the fight where his low speed due to broken legs means he's stuck Dashing instead of attacking, and a sort of broken earth type effect to create difficult terrain.

[Specter]

Give him feats, maybe. Or an innate monster ability (such as the troglodyte's stench).

[kaoskonfety]

Firing off a old few standards...

grapples or knocks prone on a hit (weirdly contorting or massive arms),
good movement (it looks lopsided and lumbering but it take HUGE paces or leaps from time to time),
reach (cause reach),
pounce/charge/rake bonus attacks (its misshapen legs hide jagged bones or claws),
a climb speed and some interesting terrain (hilarious with reach/grapples and someplace to drop PC's down into - perhaps not even for damage but they need to spend a round or 2 getting back to the fray, breaking out of nets or whatever),
envenomed strikes (nasty disease, rotten flesh breaking off in the wounds, actual poisons)

as an alternative - leave it slow lumbering and a sack of HP and make it a battlefield issue as a not worth going toe to toe with threat, a mad damage output and/or near unstoppable defences make it a terrain hazard the caster is trying to hid behind or use. But in the open without support? See the Gibbering Mouther, not getting close means you win. The portal wizard occasionally tossing you next to it being Bad News town