After each Hero's Turn, choose one of Wystra's OR the wolves' actions.
When she defeats a Hampered enemy, Creatures in Reach 3 of them must make a DC14 DEX save or take 2d6+10 Cold and become Restrained (Escape DC14), half damage only on save.
Immune to Cold. When Slowed, instead of halved, her Move is doubled.
- •Frost Axe.
Move 8, then 2d6+10 (or VS Hampered: 2d6+30).
- •Chilling Gust.
Move 4, then (Line 8) DC14 STR save or 1d6+20 Cold and Slowed, half damage only on save.
- •Death Glare (1/Round).
(Range 8) DC14 WIL save or +1 Wound and Frightened.
Wystra commands 1/Hero Frost Wolves. Each wolf can only attack once per Round.
- •Frostbite.
Up to 2 wolves Move 8, then 1d6+5 Cold. On Damage: Prone.
- •Pack Attack (1/Round).
(A Creature in melee of at least 2 wolves) DC14 STR save or 2d6+5 Cold per wolf, half on save. On Damage: Prone.
Cryostasis: When Wystra is Bloodied, she gains Heavy Armor by becoming covered in thick ice. She has Speed 0, but has Resistance to all remaining damage types except Fire. After each Hero Turn that she takes no damage, she regains 10 HP. Wystra can't use Frost Axe while in this form.
The ice shield and any remaining Frost Wolves shatter and give Wystra a final surge! 120 more damage and she dies. Until then, she loses Cryostasis and her wolves, but all DCs become 18 and all Actions gain +4 Move.
"Some call her the Red-Eyed Witch, and others the Witch of Winter. Few survive long enough to call her both." - Ralph Ignatius Peterson.
Notes:
- Wystra's On Kill/Defeat effect with Exploding Frost (intended to trigger with Frost Axe) is more reliable/dramatic when there are NPC's friendly with the Heroes involved. The intended scenario/play pattern is for the first (or first part of the) fight happens when the Heroes are with a bunch of regular NPCs (such as townsfolk or simple hunters, likely no higher than level 3 or 5). Once the Heroes realize that Wystra is extremely deadly against NPCs (resulting in collateral damage), the Heroes should draw her away from the crowds to fight against just the party. This can either be the same fight or two separate fights (fight in town, then seek out and fight her in her lair/out in the wild).
For a less grim, closer to PG version, you can flavor Exploding Frost's special effect when defeating a Hampered enemy as causing the built-up Ice magic to forcefully disperse. Instead of a gory explosion, the "dead" creature is more or less intact and able to be resuscitated or revived, depending on the lightness/severity of your campaign.
For a more grim, PG13+ version, you can flavor Exploding Frost's special effect when defeating a Hampered enemy as causing the blood in the open wounds to rapidly freeze, expand, and explode. They cannot be resuscitated or revived unless using a revival method that doesn't require all of the body.
Don't use Pack Attack unless you manage to have at least 3 Frost Wolves in melee of a Hero. Target NPCs that are near one or more Heroes to trigger Exploding Frost more often.