Creating a Wizard/Cleric NPC

[Silmeril]

So I am converting an old module into the 5e ruleset and have come to an interesting problem. I have an NPC that is 18th level Wizard and 18th level Cleric. Each has it's own list of spells and different castings. How do you seperate these spells out as well as the casting limitations (number of spells cast per level per class.) Is there a trick to this or an extension? Or is this something that hasn't been considered yet?

Thanks in advance

[LordEntrails]

5E does not do different spell slots or casts per day per casting class. Spell slots are per spell level, and your available spells are per class lists. So what you want to do isn't RAW/RAI.

First, 5E doesn't have 36th level rules, so you will have to make something up. For me, I would just figure out what you want flavor wise and not worry about some sort of feature by feature conversion. i.e. in 5E, an NPC doesn't need dozens of spells and slots, because they will never use them. A BBEG fight might last 10-15 rounds, maybe. So, what I would do; give them the spell slots of a 20th level full caster. Then give them spells from both the wizard and cleric spell list, maybe 3-5 total spells per spell level. Then give them a couple of features from each class. Then a magic item or two, or the abilities/features from such. Then give them HP based upon the CR rating rules from the DMG (not from class HP, that will be way too few).

[Silmeril]

Excellent point. I'll rethink the conversion. Thanks!

Upon further examination, You have steered me into a solution of another issue I was facing with this campaign. Thank you for the nudge!!