In my fantasy story, primitive humans are going up against a single powerful creature. They must kill the creature to gain their freedom, but they soon realize that the only way to slay the creature is by sacrificing oneself to do so. I'm trying to create a creature where such a scenario is impossible to avoid. Below are the details:
- This creature needs to be humanoid in shape. It must have hands, and be able to wield a sword.
- It can be larger than humans, but only by two or three feet max. It should not be shorter than humans.
- This creature is magical, but I'm trying to avoid magical answers to this question if I can. If there's just absolutely no way such a creature can exist with biology alone, I'll allow a very small amount of hand-waving.
- The primitive humans fight with this creature, and can avoid its attacks.
- If one of the primitive humans wants to kill this creature, they have to lethally wound it (behead, stab through chest, etc.). Bleeding out and debilitating injuries are not going to have any effect (this can be due to some sort of healing magic if necessary). Killing the creature this way has to kill the human.
- Both human and creature have to die in combat, or shortly thereafter. So no prophecies or other delays involving 'if you kill the creature, you also die eventually.'
- The human needs to die trying to kill the creature, but he also has to be successful. This is very important.
- Finally, primitive humans are going up against this creature with the simplest of iron armor or anything less effective (ie, toughened hides - not very effective). They are using a single sword.
- Note: A number of answers have assumed the creature is trying to eat the humans. This is not the case. Remember this 'creature' is humanoid, and does hold and use a sword with skill. I am using the word 'creature' here to avoid certain connotations other labels might imply.
I'm after a creature which is unkillable unless the attacking human sacrifices himself in the process of killing it. This has to occur during combat (so no sacrificial rituals or anything like that).
For example, maybe the creature can only be slain by running a sword all the way through it, but its blood is poisonous. To get close enough to kill the creature, the human would invariably get some of the blood on his hands.
The problem with this and similar ideas is that it's not fool-proof, and that is the most important part: The sacrifice has to be ABSOLUTELY necessary. There is no doubt: if the human kills the creature, he will die. Period. In the above example, one could easily go after the creature with a lance, and thus avoid the blood entirely. Such workarounds cannot be present.
So here's my question: can you design a humanoid creature which is impossible to kill, unless you sacrifice yourself in the process? Remember that the need for this sacrifice has to be unavoidable. Remember also that I want to stick to biology. If there is simply no way to get this effect with biology alone, get as close as you can, and then hand-wave the rest. But try with biology first.
This question is not primarily opinion-based. While I will allow answers incorporating some magic, I will value the biology answers more. If such a creature can be designed using biology alone, that will be better than an answer using magic. Similarly, if two answers both use magic, the one with the least amount of hand-waving and the better matching biology will be better. Answers which use just hand-waving will be ignored.
EDIT: Concerns have been raised that this question is primarily opinion based because it is about a story (despite the above explaining the exact opposite). I will explain why this is not the case:
Yes, this question is for a story (as are well over half of the questions on Worldbuilding SE). However, the question is purely a creature-design question, and perfectly on topic because of that. Yes, the question is based on a single event which happens in the story, but that does not mean I'm asking about that specific event, and how it can unfold. Instead, I'm trying to design a creature so that anyone going up against it (not just the character in my story) would have to die if they were to slay it. If am specifically not relying on plot devices to accomplish this, and instead designing a creature to fulfill this need. This is the very essence of the creature-design tag.
Also, this question is not a duplicate of this question. That question was trying to pin what I needed on combat, which was why it was closed: it was asking for a story. This question is asking for a creature design where the creature is unkillable save by sacrifice.
Post-Answer edit: I'm including this edit for future viewers. I felt that the best answer to this question was a combination of two answers. I would like to point them out to future viewers. I feel that a weak spot (Loren Pechtel's answer) is a must, as a point protected from all angles but below prevents any ranged shenanigans. However, I feel that some sort of gas (SealBoi's answer) is better than a spray, as the cloud cannot be blocked and, if large enough and deadly in any amount, guarantees death. As a bonus, an armor plate can protect the weak spot and must be pried open (Cyrus' answer), thus further ensuring archery is not an option.