We already have fiend blooded sorcerers how about one with some true power?
Child of Merlin
Many tales tell of a powerful magic user who offered wisdom to kings and guided bloodlines for centuries. Few remember that this person was a child of the most powerful devil. Merlin was a powerful man, but his diabolical heritage passes threw his bloodline to all his children and his lust for power and women meant that he has children across the multiverse. You are one of those children whose spark has awoken and now you manifest all the awesome powers that master magician with devil blood can bear!
Guide of kings
At 1st level, Merlin’s ability to guide others emerges in your. You gain the bardic inspiration ability of a bard of your level. You can use this a number of times per day equal to your Charisma modifier.
Wild Man of the Woods and Hells
Starting at 6th level, Merlin’s shape changing abilities manifest in you. You gain the wild shape ability of a druid of your level. As an added bonus, you can choose to change your shape into an animal or a fiend following the same CR restrictions as a a druid of your level.
Master Communicator and Shapeshifter
At 14th level, you gain the ability to change your shape as you see fit to match any occasion. You can cast polymorph on your self twice per day. Furthermore, you gain the permanent effect of the tongues spell to talk with all creatures who understand language.
Your Patriarchs Powers and Weaknesses
Beginning at 18th level, you gain the faults of your patriarch, but also gain a larger portion of his powers. You gain disadvantage on all saves vs any effect that causes the Charmed condition, cause psychic damage, or control your mind, but you also gain advantage on all other saving throws. You gain resistance to all other damage, but gain vulnerability vs all bludgeoning damage. For these effects and damages, apply worse penalty if multiple effects or damage types are mixed together.