If you don't want to know about Forgotten beasts then leave now.
I breached the first layer of cavern when digging out my fortress, high above the ground level of the cavern. Eventually a forgotten beast showed up, couldn't path to my fort, and wandered off.
later when I was excavating more, I cut into an invisible area, and designated a wall to be built to block it off.
About a month later, I notice about 4 of my animals are dead, so I check my announcements. The wall got suspended and the Forgotten Beast apparently had made it's way to the other side of the cavern. By this time, it's in the dining room slaughtering animals and getting attacked by war jaguars. So I send in the military. They hack away and hack away and I realize it's not getting past bruising. I watched helplessly hoping for something miraculous. Nothing.
Eventually they wandered into the refuse piles, so I sent in the most noble craftsmen of my fort, the masons. I had to deconstruct about half of the chairs/tables in my dining room and un-suspend walls hundreds of times, but eventually I walled in my 6 surviving members of the military and the forgotten beast in the middle of my dining room. Most of them are injured, and dying of thirst. The Forgotten beast has actually healed some of its wounds.
Since I will not be able to retrieve their corpses to put in a tomb, what do I build to honor their gallant efforts?
tldr: I had to wall in a few military dwarves and a forgotten beast, what do I build to honor them?