My party encountered the Balhannoth in their last game. I had a dungeon (which was actually someone's mind), which was crawling with Aberrations, and I wanted a few aberration "minibosses" in various places. So I intentionally used some legendary monsters solo for the effect, with the lead up to the big boss with his personal army of minions.
The party is 5 8th level characters and at this point are pretty drained, running very low on spells and resources. They encounter the creature invisible, who then immediately attacks them as its guarding the room from unwanted attention. The room is filled with computer banks (the mind's "math center"), and there is electricity running between them. Every time someone moves (or is forced moved) through the banks, there is a 50% chance of taking 4d6 elect damage (DC 15 reflex for half).
Overall the bard and druid polymorph/wildshape and charge the creature (the druid has scent so at least knows where it is, even if they still take the invis penalties). They both get grappled, and the druid starts taking bite damage from the main attack and legendary actions. The cleric tries to command the creature, who has an abysmal will save but whose legendary resistance kicks in. The fighter also runs on top of the banks and engages as well. Warlock has witchsight so sees through the invis, and just blasts it to hell.
Long story short I did some good damage to the druid, knocking it out of wildshape and scaring the player a bit (I got lucky on the 50% chance so when the tentacle grabbed him and pulled him in I got in some elect damage as well.). But the creature's HP are got low for a CR 11 legendary, and the party smacked it down in 2 rounds.
Overall: A disappointing showing overall, made a bit better by how low the party is on resources, adding in a fear factor. I think at full strength this wouldn't have registered as a real combat. Once again showing the old saying, no mini boss without minions!
Highlights:
1) The creature is very "vanilla". Its just grappling and damage, no special effects.
2) The creature really really relies on its invis as a defensive measure. Otherwise it has low HP and saves for its CR. So if the party can thwart that, its easy pickings.
3) I feel that the creature's schtick works against its stats. The fact that it can go invis a lot doesn't work well with its desire to grapple all the time (which basically reveals its location). The fact it pulls creatures into it goes against how squishy it is. The best way to make it interesting is to make the reel in effect combine with something (I used the electricity thing, others could use a pit or something).
4) Offensively it was alright once I got my grapple on. The 25 damage bite did solid damage especially when my legendary actions let chomp 3 extra times.