According to Upper Krust's CR guide, each additional Intellegent Undead HD grants +0.45 CR, rounded down.
Advanced Winterwight CR 27
Female Winterwight Advanced +10 HD
Chaotic Evil Medium Undead (Cold)
, Senses ; ; Darkvision 60'
Aura Cold Aura (10 ft., 2d10 cold damage, cold creatures gain Fast Healing 10/turn resistance +6)
AC 46, touch 18, flat-footed 38 (Dex +8, Natural +28)
hp 279 (42d12 HD); DR 10/-
Immune Cold, Undead Immunities; Resist Turn Resistance +6; SR 34
Spd Walk 90 ft.
Melee Claw +44/+44/+39/+34/+29 (6d8+22 DC 36 or 4 con drain/19-20/x3) Skull Butt +39 (2d6+11 DC 36 or 4 con drain)
Base Atk +21;
Space 5 ft. ft.; Reach 5 ft.
Atk Options Rend 6d8+31
Spell-like Abilities (CL 23): At will - cone of cold, control weather, dimension door, spell immunity (fireball), sleet storm (a winterwight is immune to the effects of a sleet storm spell), wall of ice
Abilities, , Con — , ,
Feats Dire Charge, Epic Prowess, Epic Speed, Improved Critical (Claw), Improved Critical Multiplier (Claw), Improved Initiative, Improved Natural Attack (Claw), Improved Rapid Strike (Claw), Power Attack, Rapidstrike (Claw), Run, Superior Initiative, Superior Natural Attack (Claw)