Pact Breaker

You lose all warlock levels but have the option to add half of the levels(rounded up) to a different class you also get to keep one of the first level spells from your warlock spells(must be one of the 1st level spells provided by your now previous patron) that you can cast once a short rest without a spell slot (uses charisma as your spellcasting ability). You also suffer a curse based off of your patron. The amount of curses you suffer are also based off of your warlock levels lost(1-5 only first curse, 6 and up both curses). The curses can only be removed via a wish spell or direct divine intervention (or something equivalent made by your dm).

Archfey: you have disadvantage on saves against illusion spells. you have disadvantage of saves against being magically charmed (reduces charmed immunity to resistance and so on and so forth)

Celestial: you have weakness against radiant damage, and healing spells heal you for 2 less

Fiend: you have weakness against fire damage, your constitution is lowered by one and your maximum constitution score is lowered by one

Great Old One: you have weakness against psychic damage, and have disadvantage on saves against magical means of being frightened

Hexblade: you get critted on a 19 or 20, and a specter is periodically sent after you to hunt you down(the intervals and behavior of such specters are completely up to your dm).

Raven Queen:you have weakness against necrotic damage, and a 2 on a death saving throw counts as a one.

Undying: mindless undead will go out of their way to attack you in combat, also if you roll a 20 on a death saving throw it doesn't heal you it just counts as a regular success. 

Seeker: one npc starts to hunt you, and scrying using you as their target causes the spell to always succeed.