Zardnaar
Legend
High level encounters are always hard to design and 5E doesn't have a great variety of higher level monsters.
My advice is throw the encounter guidelines out, use large numbers of lower CR critters (20 Ghouls, 40 Kobolds, 12 purple worms), use spell casters (in multiples). 4 invokers riding a dragon or wyvern each (hitting the PCs with 4 fireballs, chain lightnings etc), use multiples of high CR critters (4-6 beholders as part of a hive), use poison, use very nasty traps with spells like power word kill keyed on them. Put spells like foesight on NPC champions/important PCs.
Basically pull out all the stops.
My advice is throw the encounter guidelines out, use large numbers of lower CR critters (20 Ghouls, 40 Kobolds, 12 purple worms), use spell casters (in multiples). 4 invokers riding a dragon or wyvern each (hitting the PCs with 4 fireballs, chain lightnings etc), use multiples of high CR critters (4-6 beholders as part of a hive), use poison, use very nasty traps with spells like power word kill keyed on them. Put spells like foesight on NPC champions/important PCs.
Basically pull out all the stops.