Wystra commands 1/Hero Frost Wolves. Each Frost Wolf has 50 HP and is Unarmored. Wystra can use subsequent Turn to have a wolf Interpose for her if within Reach 3. When a wolf is dropped to 0 HP, they die and can no longer be called upon. When all wolves die, their Actions can't be chosen anymore.
Immune to Cold. When Slowed, she gains 2x Speed instead of having it halved.
- •Frost Axe.
Move 8, then 2d6+20, Advantage VS Hampered.
On Hit VS Hampered targets: +10 damage.
On Kill VS Hampered: The target's blood freezes and explodes, and Creatures in Reach 3 must make a DC14 DEX save or suffer 2d6+10 Cold and Restrained (Escape DC14), half damage only on save.
- •Chilling Gust.
Move 4, then (Line 8) DC14 STR save or 1d6+10 Cold, Slowed, and Knockback 2. Half damage only on save.
- •Death Glare (1/Round).
(Range 8) DC14 WIL save or +1 Wound and Frightened.
- •Wolfbite (1 per Wolf/Round).
2 Frost 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/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 Heavy Armor and its Resistances, 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 effect 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 reflavor the Axe's special On Kill effect 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.
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.