D&D 5E (2014) How To Make High Level 5E Work.For You +

[Zardnaar]

Sorry, should have linked your name to it.

I have descent into Avernus. Haven't run it though, might now that group finished the Planescape adventure. First part looks REALLY linear and then it seems to open up.

I might just take the end bits and adapt to my, now 18th level, group.

Have a look at the Bhaalists. Specifically the unstoppable mechanic.

Look at Rakshasa in 5.5 MM.

Marilith as well. Specifically its reactions.

Look at Mythic mechanic in Theros and Fizbans.

See where im going with this? 5E has the ingredients already.