He had been injured by a goblin ambush the previous season, and then went to the fort on his own two feet (yes, he had both); disabled all jobs off doctors and forgot about it. Now I just noticed my pop had one less, Mr. Adon can't be located in the [ u ]nits list, be it living or dead, I haven't seen his corpse anywhere, and there are no mysterious clouds of miasma floating around.
Wait for it to say he's been missing for a week, or if you've already had the announcement, make a slab and engrave it at the Crafts Workshop, when you place it you can examine it (I can't remember if you examine it using k, t or q) and determine the exact cause of death.
Apparently Cave Dragons can dispatch some Forgotten beasts with ease. Who knew?
I've had Titans made of water, and Forgotten Beasts made out of salt, they generally don't live very long. The only cavern creature to kill more than one of my Dwarves directly was a Cave Dragon, in about the 2nd year of Yelledwheeled my 2 mechanics at the time were loading cage traps in cavern 3 when they were attacked and killed by a Cave Dragon, fortunately, their deaths likely saved many others, as they managed to get one of the traps set up before the Cave Dragon could go into the fort, and the named CD has been sitting in a cage ever since.
Speaking of Yelledwheeled (which I just noticed I've written wrong in every other post, whoops) I've cleared out almost the entire bottom floor of the 1st Cavern layer (well, the level above where the water is, I'm flooring over the water for now, and pretending it isn't there), soon I'll set-up some ballista to deal with the water death trees/shrooms so I can create a safe building environment for the
slaves Cheese Makers builders as they put in the other floors and walls on the remaining 2 layers between. After that's done, I might even give them a bedroom expansion... under the stipulation that they manage to get it done with 0 work related casualties.