agnus the Frost keeper

Magic resistance.  advantage on saving throws and other magical effects

legendary resistance. 3/day

legendary ability.  2/day

Ice stride. 1/day magically move through the ice 60ft

ice palace. 1/day agnus summon may a ice castle it has the same stat block (but double hp) of instant fortress the castle will contain 4 frozen eyes which she can see and cast spells out of.

orb of spellcrest. (passive) agnus has a orb that magically hovers around her and casts a version of frostbite as if it was lvl 18 (4d6) con save dc 21

icy reflection. 5/short rest agnus upon being hit with a ranged or spell attack may use this as a free action to have a 1/4 chance of randomly deflecting the attack to a random target 

spirit of the north B-.  (passive) agnus will heal 15 hp at the start of their turn

final hope B+. after agnus loses at least 70% (average 543) hp they will gain the ability to take 3 actions a turn 2 actions may be spent casting spells ignoring the leveled spell rule and one may be either frozen barrage or The icicle and agnus will set their ac to 25

call to the artic. 1/day agnus may summon either 4 cr 3 or below Feys or 2 cr 6 or below Feys or 1 cr 10 or below Fey 

frozen spirit. agnus always has chilled shield cast if it is dispelled it will come back after 2 rounds 

Spellcasting. The agnus is a 19-level spellcaster. Its spellcasting ability is wisdom (spell save DC 28, +18 to hit with spell attacks). The agnus has following druid/ice spells prepared:
Cantrips (at will): anything ice based
1st level (4 slots): shield, command, cure wounds, healing word
2ndlevel (3 slots): [hold person], [mirror image] 
3rd level (3 slots): [ball of ice (fireball)], [mass healing word], [revivify], [counterspell]
4th level (3 slots): [ice storm], [polymorph]
5tht level (4 slots): [wall of force], [cone of cold] 
6th level (2 slots): [heal], [harm], [wall of ice]): 
7th level (2 slots) [force cage], [teleport] 
8th level (2 slots): [feeble mind], [power word stun], [polar ray]
9th level (1 slots): [storm of vengeance], [shapechange]

The icicle.

Action Melee Attack. Melee Weapon Attack: +12 to hit, reach 5-10 ft., 1 target. Hit: 12 (3d8) [magical piercing] damage. + 12(2d12) [cold] damage.

Frozen barrage 

Action Ranged Attack. RangedWeapon Attack: +8 to hit, range 10/100 ft., 3 target. Hit: 6 (1d6 + 3) [magical piercing] damage.

this creature can use 2 bonus actions per turn

agnus can cost cone of cold as a bonus action (limited to 3 per long rest) and it is not counted as a leveled spell.

agnus can cast rimes binding ice as a bonus action and it is not counted as a leveled spell

The icicle . +12 to hit 3d8 magical piercing + 2d12 cold reach 5-10ft 

blinding ice. a creature must make a dc 17 wisdom save or be blinded until the end until the end of that creatures next turn

agnus gains 1d4 +2 legendary actions per turn and all legendary actions they did not spend will be erased. these legendary actions may inflict freeze.

freeze is a homebrew condition a creature may roll a con save (dc 25) at the end of its turn to get rid of 1 stack of freeze. for each stack of freeze the creature will lose 10ft of movement speed 

at 3 stacks of freeze the creature will lose its bonus action  

at 5 stacks of freeze the creature will lose its action

at 10 stacks of freeze the creature will be frozen and take 100 (1d100 +50) cold damage per turn it will also be unable to move or cast spells 

 The icicle strikes back! (Costs 2 Actions). agnus will make a single attack with the icicle melee weapon

polar chill (Costs 3 Actions). agnus will make a spell attack with polar ray (this is only available if agnus still has 8th or 9th level spell slots and will consume a spell slot) and the creature must make a con save (dc 22or take 1 stack of freeze

the cold is biting, isn't it? (Costs 1 Action). agnus will choose 1 creature that creature must make a dc 18 str save or be restrained and take 10(1d10+5) cold damage

chilly, isn't it? (Costs 4 Actions). agnus will make 2 creatures make a dc 25 con save or take 20(2d10+10) cold damage and 1 stack of freeze 

The Last Winter... (Costs 6 Actions). agnus will choose 3 creatures those creatures must make a dex save dc 22 or take 50(5d10+25) cold and a con save if they fail the first save, they must make a con save dc 25 or take 3 stacks of freeze