Not gonna lie, "magic stealer" is a really bad name. What's wrong with the classic spellthief?
I might be thinking of 3.5, but isn't there a risk of AoO when you use a Spell when engaged?
There's no such thing in 5e/5e2024.
Closest thing is a feat that player characters can take, which allows them to make one opportunity attack when a caster casts a spell.
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