CR of BBEG for lvl 8 players?

Hi, 

I have a party of 5. They are level 8, and are coming up on the BBEG, who is a spellcaster. I have monk, rogue, bard, fighter and sorcerer. I really want to use the Arch Hag stats from the 2024 MM, is that way too hard? My party has a few +1 magic items, and just between the three martial classes I have they probably deal 50 damage a round. I could have the party have an ally if needed or I could change the stats a bit to make it a little fairer. They would only face the hag, and they would hopefully have a full rest and some health potions.

If anyone has had level 8 players, what CR was your BBEG?

CR 21 would destroy your players. It would probably kill off one to two of them in a round alone, and looking at it's AoE, it would be pretty nasty.

I would instead use a different kind of hag, or skin a spellcasting enemy as a hag, like the Noble Prodigy. Your party is well balanced, between the bard, sorcerer, and fighter alone, they would be able to get things done, not mentioning the extra support the rogue and monk can offer. I would recommend running a spellcasting enemy skinned as a hag, seeing as most hags are too low CR to function as a BBEG. Throw in some minions to act as meat shields and some dynamic terrain for the BBEG to exploit.

Example: Noble Prodigy skinned as a powerful hag with sorcerous abilities, supported by two lesser hags in a coven, such as Beaur or Annis hags. The lesser hags take the front and distract the players in close range while the BBEG rains fire down on them from a covered position.