Progression Summary
Witchery: Increases on levels 14, 17, 20 to a max of 12 per long rest
At level 11 an additional 6th level spell slot will be granted
New spells can be learned and retrained at each level.

13: Craft Feature - Mastery Curse
14: Craft Feature
15: Bewitching Selection
16: ASI
17: Bewitching Selection
18: Craft Feature
19: Bewitching Selection
20: ASI

Bewitching:
As the game does not support higher level spell slots than 6th, I've opted to utilize a very Warlock-y system instead.  At levels 15, 17, 19 you gain a selection of an ability called a Bewitching. These are powerful spells inspired by witches across myths, you may also retrain out of a Bewitching whenever you gain one.

• Snedronningen's Frozen Realm - Conjure a frozen landscape that deals cold damage and can freeze enemies.
• Locasta's Travel Blessing - Bless your allies and enable them to move freely using a bonus action.
• Bellflower's Reversal of Fate - Heal your allies and grant them shielding against farther harm.
• Grimhilde's Caustic Detonation - Summon trapped apple baskets that explode when damaged.
• Kirke's Metamorphosis - Polymorph hordes of enemies into harmless sheep.
• Jadis' Words of Reckoning - Deal extreme force damage to enemies.
• Weatherwax's Applied Headology - Induce advantage to your allies and bane onto enemies.
• Nyneve's Blade of Victory - Summon a blade of victory that provides protection.
• Angrboda's Ironwood Sanctuary - Enshroud allies in ironwood that make resistant to all damage and spells.
  

• 13: Embody: Enhance Ability as a curse.
• 14: Servants of the Three: Summon 2 allies aligned with the Three in One.
• 18: Phosphoros Enodia: Conjure a gate that bestows grand blessings upon allies and devastates enemies.
  

• 13: Invigorate: Heal and grant allies temporary hp.
• 14: Wyrdwood Auspices: Enchant allies with Wyrdwood Magic that protects either their body, mind, or soul.
• 18: Wyrdwood Manifest: Conjure a portion of the Wyrdwood to douse fires, grants regen to allies on plant surfaces, makes allies immune to difficult terrain, and applies disadvantage to enemies.
  

• 13: Decry: Applies Bestow Curse: Dread to enemies.
• 14: Malicious Wind: Deal high psychic damage while applying a 3 stage curse that makes enemies poisoned, baned, and blind.
• 18: Rot in Rancor: Makes your next spell apply Spell Rancor and Attack Rot, making targets take necrotic damage when they take an attack or spell action.
  

• 13: Belittle: Applies a debuff that subtracts 2d4 from enemy attack rolls and reduces movement speed.
• 14: Guardian Possession: Apply Death Ward and temporary HP to an ally. While temporary HP is active retaliate against attackers.
• 18: Parade of Wraiths: Summon wraiths that heals and makes allies immune to damage (1 turn) and frightens enemies while making them vulnerable to Force, Necrotic, Psychic, and Radiant damage.
  

• 13: Reinforce: Applies Hero's Feast to a summoned creature, also increase each of their physical abilities by +2.
  

• 14: Witch Haste: Grants an extra Action Point and Bonus Action Point temporarily.
  

• 18: Sovereign Summon Boost: Grants summoned creatures advantage on attacks, saving throws, lower critical threshold, and an extra action point.