We were a group of seven people, I was the dungeon master at that time and one of my best friends was a hobgoblin paladin, we also had a wizard, a bard, a ranger a psion and a guy who constantly changed his character because he kept on dying over and over again. My hobgoblin friend who at some point befriended a little baby mimic that he found in a chess board (the mimic took the form of the chess pieces) was so happy with his mimic that he learned deep speech in order to talk to him, I really can't remember why he liked it but he managed to convince me and let him keep it. So the group had a mission and had to travel to the shadowfell, there they had to infiltrate a tomb inside an abandoned castle, but the castle was full of traps. They inspected everything around before even trying to enter, it was then that a character found a hole in ground next to the castle walls, they knew they had to look for a tomb so some of them thought it would be best to slip down the hole than enter through the main gate. The castle was part of a greater campaign and at this point we had been playing for 8 hrs or so already so after they found the hole, they put up a tent, the bard made use of make shelter and we ended the session.
The next week my hobgoblin friend couldn't play with us and since the day he got the mimic, he asked me if the mimic could help in battle, I said yes because I saw no inconvinient, now all this played an important part on what is going to happen. My friend told us that night that he would leave the mimic in charge of the whole team, and he asked them to take good care of it.
Moving forward to the next week, the session started and we decided that the hobgoblin would go back to the guild because he forgot something, as an excuse for him not being present at the moment. The first one to enter the hole was the bard, it was played by a guy but the bard was a teen girl that fear nothing because she thought she was the ultimate bard and nothing could stop her, so following the character's personality she was the first to go down, no less than 100 ft into an underground cave, a very large cave. They took no precaution at all and the rest of the team couldn't communicate with him so the next thing they did was throw the mimic down the tunnel and then the rest followed just to arrive insive a large cave full of bones of different size and age which in fact was the nest of a powerful hydra that only had two heads at the moment.
The hydra was sleeping and my players, all of them failed their stealth checks, it was hilarious as one of them even rolled a 1. So the hydra woke up and they rolled initiative, more low numbers and the hydra got a high initiative. It started to attack the bard since she was the closest one, the bard was next and she thought she would be safe by leaving because she was not next to the hydra but its reach was enough to hit her with two opportunity attacks, the bard was already dying, the others helped her and fought the best relentlessly, however the more damage the hydra received the ranger managed to cut off both heads and two more heads started to grow and attack the players, then other two, the hydra now had four heads.
The fierce fight continued for an hour or so, the wizard launched powerful spells, the ranger kept trying to slash the beast down and everyone was helping eachother but my players kept on making lots of mistakes, this was the first time they ever fought against a hydra but they were no rookies by any means, so they decided to run because the bard ran out of healing spells. All of them managed to run away from the encounter, all but the mimic which turned into a rock in order to survive.
They found a hidden door inside a hole on one of the cave's walls, they escaped and went through that door, they did their best to get out of the castle and after three or more fights they managed to get out of the castle, this time they left trough the main gate. Exhausted they camped in the same spot and the bard used make shelter again, the session ended there and the next week our hobgoblin paladin was back into action. The first thing the team said to him is that they lost the mimic inside the cave of the giant Hydra. The paladin was furious, they left his beloved pet inside a nest of a very dangerous beast! The paladin controlled himself and started to explain why that was a dick move, how mimic was part of the team, just as any dog or cat or any animal a family would have by their side, the mimic ate with the team, slept with the mimic, even cared for them and then he rolled a natural 20 plus bonus on Diplomacy. No one had any words to say to him after his emotive speech.
They got ready and raided the Hydra's nest once again. This time the hydra was more dangerous since it had all its heads available since the start of the battle, my paladin friend called out for the mimic, he got it and put it inside his bag of holding, he planned an escape route from the battle and while he recovered his pet, the team helped him by keeping the hydra busy, but the cost was too high, the team ran low on hp and when the paladin turned around to look at the party, the hydra bit the bard's head, the other heads bit her limbs and they all got a piece to eat. This was a red flag for the hobgoblin who ran away from the battle and left the team behind as revenge for leaving the little mimic to its doom. The hydra's attacks got fiercer and it devoured the whole team but the paladin and the mimic.
Everyone in the table was surprised, he gave them a ted talk and a very good speech so they could all go and rescue the little mimic but in fact what the hobgoblin wanted was revenge on the team he could no longer trust because they abandoned a member and used it as bait to escape. Everyone understood they did wrong and didn't get mad at the paladin, so the paladin went back and searched for someone who would help him on his mission, everyone had to make new characters and joined the paladin on raiding the castle.