9th level spellcaster, but only 2 5th level spell slots and 5d8 HPs?
Probably the least interesting aspect of the concept, but I understand how the numbers might make you curious.
To start with, I was looking at the AD&D sage guidelines (see spoiler below), and later the Warlock NPC stats in Volo's Guide to Monsters.
[SBLOCK=AD&D sages]
[/SBLOCK]I wanted to create a NPC who could accompany the PCs (e.g. on a "protect the sage to the distant library" mission) without a great chance of being one-shotted by a monster... so with hit points comparable to a Druid (5d8+5) or Priest (5d8+5)... instead of a Commoner (1d8) or Noble (2d8).
Certain spells model aspects of a sage's knowledge very well (e.g. identify is a great short-hand way to refer to a sage's ability to appraise magical relics of the past, whereas legend lore is a great way to model the sage recalling excerpts from cryptic texts). However, conceptually a sage shouldn't be throwing down a lot of magic all at once, but what they do know should be replicable fairly regularly (the sage can spout lore all day long)...and that made me think of warlock spellcasting.
I knew that the 5th-level legend lore had to be available to the sage, for thematic reasons. Hence I drafted a 9th level spellcaster emulating a warlock, because 5th level spells are available once a spellcaster has 9th level spellcasting ability.
Now, at that level a warlock would cast all spells using 5th level slots. While most of the spells I chose for the sage can't be upcast, the sage can cast dispel magic at 5th level. That's an important thematic consideration, because it means the sage can break spells like dominate person, geas, hallow, planar binding, scrying, teleportation circle, and a few others which have considerable narrative impacts. This opens up adventure scenarios where, for example, the PCs need to reach a sage who know how to break a fey geas cast on one of them, or need to consult a sage who knows the Celestial seals needed to break an unholy influence gripping ancient ruins.
I was a bit concerned that being a 9th level spellcaster with 5d8 hit points might confuse a DM, but there is official precedent of these things not being the same with the Warlock of the Fiend (17th level spellcaster with 12d8+24 hit points), not to mention others like the Cult Fanatic (4th level spellcaster with 6d8+6 hit points). Moreover, it's well established in 5e that NPCs are not created with same rules as PCs.
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