You have lived too long, learnt too much, and practiced hard.

After a long life of training and improving yourself, you are turning into the BBEG you have feared.

You increase 2 of your ability scores by 1, gain proficiency in two additional saving throws of your choice, gain proficiency in three additional skills and double your proficiency bonus for two of your existing proficient skills.

You also gain an additional attunement slot for any item of your choice, gain advantage on initiative rolls if you do not already have it, and can choose 1 additional feat from the feats list.

Unfortunately, your soul has also become corrupted by living too long without consistent friendship or family. Your alignment become either neutral evil, chaotic evil or lawful evil. There is a sickness in your heart that will cause any resurrection attempts made on your soul after your death fail, unless they raise you as an undead or construct. 

You learn the infestation cantrip, which you can cast at will, ignoring somatic and material components, to a range of 60 ft. The parasites that appear as part of this spell resemble aspects of your own visage; for example, they may appear to be made of your skin, your hair, or your clothes - or they may appear as replicas of your own eyes with tiny legs. You can choose whether your infestation inflicts poison damage, acid damage or necrotic damage.