Ano, Undead Mastery requires 7D and causes 700! fatigue to the caster (if I'm wrong, please correct me). I think your starspawn wasn't able to cast it below 200 fatigue and he just hold a turn. Then the other one casted VoR as planned.
I'd really like to see UM in action. Are you going to try again next turn?
It's controversial. Some posters say they've tested and verified that a caster will never cast a spell that takes him above 200 fatigue. I've never had a problem with it. I don't know what to make of the discrepancy.
-Max
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Bauchelain - "Qwik Ben iz uzin wallhax! HAX!"
Quick Ben - "lol pwned"
["Memories of Ice", by Steven Erikson. Retranslated into l33t.]