I have had a first-hand experience with a Tentacle Demon pit, complete with a Spirit-o-Fire.
I accidentally drilled a up/down stairway right into their pit, which unleashed a swarm of 40 refugees (This pit didn't have any cages, which lagged my fortress to the Pits and back as they rushed to get to the surface) and 30 tentacle demons... plus a spirit of fire.
I ended up losing my entire fortress of 70 guys. Plus a reclaim group of 70 guys.
The tentacle demons were worse for me. I hate using military (I trap-spam.), so all the dwarves in the fortress became wrestlers. With two arms. Tentacle demons have 8. Even with four guys on one tentacle demon they only managed to wound one of its arms.
Then the other 29 showed up, and casually wrecked my fortress in the glory of 1fps. The spirit of fire took it upon himself to burn several marksdwarves and hammerdwarves that had started pounding on a single tentacle demon.
When the reclaim came, the spirit of fire decided to show up on top of the wagon. Thankfully this was after I had moved my guys back into the fort. More marksdwarves managed to shoot it out of the sky. It must have taken at least a dozen arrows, and taken out three guys, before dying.
However, it was funny because some of the tentacle demons fell into my magma pit I was using for a forge. As did some refugees
Anyways I'm on a new map now (Which nearly lost all of its guys to a goblin ambush-spam (Four groups of ambushes, including goblin-archers) in the middle of a giant engineering project... lost all the miners and masons including the mayor.) and there were 30 frog demons and a regular demon inside the pit. I ended up having to kill them all with Tweak because their pathfinding (combined with a 20z level natural cliff) were lagging me severely.
In short: Tentacle demons melt but can wrestle with the best of them. Spirits of fire fly and are therefore very dangerous. Refugees from the underworld are laggy.
I wonder what determines the demons you get >.>