how do you do athmospheric bossfights right in gritty campaigns

>>54149541
Its good to establish a soft series of calling cards associated with whatever is antagonizing the players, then have those calling cards begin to culminate towards the conclusion.

Another useful thing to do is have your big-bad open with a massive attack of some kind that should kill lesser opponents. That will help set the tone for how serious the fight is.

For example:
>group has been tracking a death cult bent on destroying the world.
>there are a few leaders of this cult they have slain who have been brought back by cursed nails driven into their foreheads.
>They later learn that these nails were created by a long dead civilization to stay the wrath of the Gods, and it is the cults goal to wage war against those Gods.
>In the final encounter, at the top of a pyramid excavated in an otherwise empty tundra, they meet the archfiend responsible.
>He is some sort of massive homunculus, pinned together with dozens of those cursed nails.
>Gives a good'ol BBEG speech, then removes a nail holding one of his eyes shut.
>The sight of his eye prompts saves against each of their stats and an a save against fear with a debuff applied for each save they failed previously.

Having that kind of setup and hamsgringing allows you to have an otherwise mundane fight that has weight. That boss wasnt particularly overpowered, though he was more powerful than anyone they had faced. His presence came from the infamy that had built up around him, the clear meaning of his form, and how much he damaged the whole party so casually.