At 20th level, the Path of the Zealot barbarian (Xanathar's Guide to Everything, p. 11) can rage indefinitely by virtue of starting a new rage just prior to their initial rage ending. This is confirmed by this question: Can I start a new rage before the previous one ends?
At 14th level, they also picked up the Rage Beyond Death feature, which lets them stay active at 0 hit points and regardless of how many death saves they've failed:
While you’re raging, having 0 hit points doesn’t knock you unconscious. You still must make death saving throws, and you suffer the normal effects of taking damage while at 0 hit points. However, if you would die due to failing death saving throws, you don’t die until your rage ends, and you die then only if you still have 0 hit points.
At 15th level, barbarians gain the Persistent Rage feature, meaning their rage ends only if they fall unconscious or choose to end it.
Given that the Barbarian can constantly remain in a state of rage, how can I ever reliably kill them?
"Reliably" meaning they won't get a save to negate the effect, since they would be able to constantly cycle their Fanatical Focus feature to ensure rerolls on saves.
Best Answer
Brute force won't work, but any other effect will work, provided it is hit point-dependent
At first blush, the 20th level Zealot Barbarian seems unkillable, and this is true if you play the Barbarian's game by engaging them in direct melee combat. Indeed, by RAW the Barbarian could massacre an entire army of mundane soldiers and continuously maintain their rage until they were able to find any sort of healing to recover the minimum 1 hit point necessary for them to keep on trucking.
However, 5e has a handful magical effects that can be used to reliably handle this situation, though.
This is a fairly short list that curiously hits both ends of the spellcasting spectrum, but either will stop the Zealot on the spot.
There are a few other options accessed by turning into a creature with true polymorph or some other method to access them. These all require you to reduce the Zealot to 0 hit points, be aware that it can be difficult to do this because the Zealot can make several saves due to Relentless Rage to instead be reduced to just 1 hit point:
These methods are most viable after you've reduced the Zealot to 0 hit points since you won't trigger their Relentless Rage, which the Zealot can probably reliably make 2-3 times before it starts getting tricky. I've avoided effects that rely on trying to reduce the Zealot's maximum hit points to 0. While they technically would work, you would find yourself spending an absurd number of rounds trying to reduce a feasible 275 maximum hit points.
One last method that would reliably work is: