Converting a 3.5e Demon Lord stat block to 5e

Question

I'm DMing a party of five and they're going after Yeenoghu as the campaign's final boss. I've looked online and haven't found any 5e stats for the Demon Lords, but I did find this for a 3.5e Yeenoghu. I've read the official conversion reference, but that's more for player characters and might not apply to extremely powerful beings like this.

I want to create a stat block that would be just as challenging for a 20th-level 5e party (with some magic items and divine blessings) as the block linked above would be for a geared-up 20th-level 3.5e party, but I haven't played 3.5e so I don't have a sense of what characters are capable of there. Obviously a +54 to hit and 45 AC is ridiculous in 5e, but using the quick conversion guide in this question, the resulting Yeenoghu seems too weak for someone with a power level similar to that of the gods.

Are there any official guides for converting very high level stat blocks between editions? Or just a published 5e stat block for Yeenoghu? According to this question, Fraz-Urb'luu has one, but I don't own the relevant book to check it for other Demon Lords.

Answer 1

A statblock for Yeenoghu is included in Appendix D of the Out of the Abyss campaign book.

You may find this statblock a bit weaker than you prefer; it's certainly not at "a power level similar to that of the gods." (Demon Lords, aren't, generally, nearly as powerful as the gods.) I'd recommend simulating a battle with your party and seeing if it's as challenging as you want it to be, and making some adjustments based on what you find.

Answer 2

Start from scratch

20th level 3.5e characters have near godlike powers. For example, a +30 or more attack bonus would not be unusual: that would hit AC 40 about half the time.

20th level 5e characters are not in the same league. An attack bonus of more than +11 would require magic or a score over 20. This would need a critical to hit an AC 40.

Look at 5e stat blocks for CR 20+ and take your guidance from those. Use the 3.5e stat blocks to decide the feel of your demon lord.