After a few hours of travel in the sleigh, you spot the castle on the horizon.  Suddenly, a strong gust of wind slams into you.  You look to your right, and notice...

Wings...

Lots of wings...

Undead Frostwyrms (10 of them!)

Name: Undead Frostwyrms x 10
HP: 14 each
AC: 14
Initiative: +1
Attack: +10 vs. AC (They attack in a pack, and the amount over your AC is the number of Frostwyrms that hit in that round.  For example, if your AC is 19, and the Frostwyrms roll a 14, when you add +10 to hit, it means that 5 of them will hit this turn: 24 - 19 = 5)
Weapon: Ice Blast (1d4 to each PC) - so if 5 Frostwyrms hit, as in the example above, damage is 5d4 to all PCs

Instructions:
This might be a good time to try out those new weapons :)