Story Weaver:
I'll say yes, and think if I need a possible consequence for that roll.
So... he... can try it now... but... you're gonna thing about... what it'll mean if it ever comes up in the future...? (Including, I suppose, if Ravnar blows it this next time...?)
If I may make a suggestion...?
This is something I worked out for another solo game I run...
Cast A Spell
When you unleash a spell, roll 2d6 and then add your modifier:
On a 10+ the the spell is successfully cast, and you may cast it again, later (the next round, if you want).
On a 7-9 the spell is cast, but choose one:
--You draw unwelcome attention or put yourself in a spot. The GM will tell you how.
--Your casting wears you out a little; take -1 ongoing to cast any spell until the next time you rest for at least an hour.
--After it is cast, you cannot cast the spell again until you rest for at least an hour.
--The spell is somehow less effective than it was supposed to be, or has some other unexpected side effect (the GM will tell you exactly what).
On a 6 or below, the spell is cast,
but the GM will choose one:
--You draw unwelcome attention or put yourself in a spot. The GM will tell you how.
--Your casting wears you out a little; take -1 ongoing to cast any spell until the next time you rest for at least an hour.
--After it is cast, you cannot cast the spell again until you rest for at least an hour.
--The spell is somehow less effective than it was supposed to be, or has some other unexpected side effect (the GM will tell you exactly what).