Introduction to Annihilation isn't as relevant with no more Iona in the format, but it's still a cool card.
Baleful Mastery is a serious consideration for the removal suite. Not confident about what I'd cut for it though.
Oriq Loremage is cool, but I don't think I want it at higher than 3 mana.
Professor Onyx - 6 mana? Oof. It's cool but way too costly.
The Biblioplex - Probably not quite good enough but fun to theorycraft.
Commander 2021 Review:
Cunning Rhetoric - I wish this was a bit stronger, but I like the style of the card.
Keen Duelist - If only it fit better! It's so good but yet so painful. Consideration all the same.
Stinging Study - At 4 mana, I'd seriously consider snatching this up. At 5, it's not quite worth it to me.
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Not saying it's not worth a slot, just trying to think about where it fits in the deck.
Edit: It's probably also worth considering that if you name black to improve Xiahou value then you blank your Snuff Out and Slaughter Pact.
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You're right that it's expensive but with rituals it won't be that hard to get there. It might also need some support in the form of grim monolith and mana vault.
Thanks for the primer anyway, I've been playing Xiahou for a year or so now and used your primer a lot when I was first doing research. A lot of useful thoughts and information