Back about two years ago, I had modded a few things in to magma to make it a little more dangerous. There were Giant Fireflies which would fly out of the magma and shoot fireballs from above (and had to be dealt with or the magma had to be capped, but they were a nice effective magma deterrent). There were also some 'Bomb Crag's, a creature made of magma with a rock shell. I wasn't good at creature modding yet, so I had taken hints from a variety of creatures made of nothing but non-flesh materials, such as Magma Man, Iron Man, and Bronze Collossus.
For many games, the creatures acted well. They were somewhat skiddish, but could occasionally, if one wasn't careful with their magma, get into the fort, and would cause havok. But I never got to see their true power, until one day, I had my dwarves collecting Glass Slug silk by the mouth of the volcano, and one of the Bomb Crags accidentally got cornered by two dwarves, and fought back. The result was that the dwarf died, and the Bomb Crag became a named creature out for living blood. I had made them very slow, but that made it all the more ominous as the creature made its way ponderously toward my fort. "It's okay", I thought, "I'll just get my military ready for it, and it will just end up dying in my long hallway of traps - the one that takes out entire goblin sieges".
To my surprise, though, the human caravan showed up, and was closer to the creature. "Ahh, perfect!" I thought. "Their experienced fighters will deal with this problem, or at least weaken it!". The caravan approached, and the Bomb Crag moved in, and the caravan fighters advanced on the creature. but BAM! A goblin ambush ambushes the creature! "Perfect", I thought, "the goblins will take care of this menace as well!". The goblins surrounded the creature and an epic battle began... Just as a Goblin Siege appeared! Yes, on top of ambushes, a siege! And I thought "Well, this is even better!" as the goblins made their way toward the Bomb Crag. NOW the TRULY epic battle began.
By truly epic, I mean the goblins were lain waste to, their corpses littering the area following their inevitable retreat. Not only had the creature taken out several goblin ambushes, it then proceeded to take out an entire goblin siege, and in the process the creature - not the goblins - took out the human caravan as well, wagons, fighters, and all. And the creature was bruised.
The creature advanced on the fort once more, and I thought 'maybe if I get a lucky hit in, or maybe if the traps are enough...'... It didn't have trapavoid, but I hadn't put up any cage traps (which ironically could have fixed this whole thing). It entered the trapped hallway... and suddenly an ambush of Dark Seethers, an arachnid race that I HAD given Trapavoid, to basically make it so one couldn't necessarily protect the whole fort with just a hall full of traps (in other words, to make up for what goblins lacked). They had encountered the creature in the hallway and were now visible! Their web paralyzes the creature, which triggers even traps it is already standing on, and the creature moves all around pursuing the dead and dying Dark Seethers that it left in its wake. The creature was hit with so many traps so many times that about half the traps in the entire hall were jammed by the time the whole thing was over. I checked the creature's status. Bruised.
It advanced, and reached the end of the traps, where it began to frighten and then slaughter dwarves like hot cakes. I checked its status. Entirely healed.
The problem was I had accidentally, in making an amalgam creature, made something with all the strength, size, ect. of a Bronze Colossus, but with all the healing abilities of an Iron Man, and made it out of a non-breaking material.
I saved that fort, though, by taking all remaining dwarves into a room, digging a long circular passage with legendary miners, walling it up behind, and then walling up the other end when we broke back into the fort. The room and tunnel remained untouched for the rest of the fortresses relatively long life.