With this dungeon level since magic attack obviously hurt the leader if he is lower than his max hitpoints when the mask breaks does he get reduced to the number of hitpoints that the second number is or do you reduce his hitpoints by the difference of the mask to the normal leader, thereby allowing him to die just by breaking the mask?
ex gold level leader has 21 hitpoints with mask 13 without
if he has taken 13 damage from magic attacks, when his masks breaks would he be dead or at 8 hitpoints?
A keyword search on 'mask' come up with this thread as the first half dozen hits...
Now you need to work through the thread a little, because there are some other questions discussed there as well, but it's all good learning.
Check this post.
The 'proper method' mentioned was quoted earlier in the thread and is from DJitD pg 11
For each wound a monster suffers, the overlord player places a wound token from the pile of unused tokens next to the monster figure on the board. A monster figure is killed when the number of wound tokens it has
received equal or exceeds its wound rating, as indicated on the monster’s reference card.
In your example when the mask breaks he will be killed because the number of wound tokens he has received will equal or exceed his (current) wound rating.