I find people overlook the Undying a lot. I've never seen him played but W20 and the ability to use it as fate is great, I also notice some ringwraith are slightly better at magic than other the Undying being one of them so he should have no problem as a support wraith, sadly his special rule does require other casters

But just because he gets the extra special doesn't mean he's not good without others, just that he's optimal when another spell caster is there

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Him and the mouth of sauron or other cheap spell caster would be cool.

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Imagine him with a mini-ringwraith who costs as much as the mouth of sauron and is just casting his spells with 2+ or 3+ while the undying has lots of dice to do command and the other difficult casts.
Or with Kardush (besides I would never play an orc throwing fire balls) who kills an orc a turn and you have as many will as you have orcs.