Forbidden Magic

Through study, dark pacts or curious experiementation you have found a way to tap into magic that was otherwise beyond your grasp.

You gain the following benefits:

• When you learn a new spell, or replace an existing spell you knew, you may chose to learn up to 1 spell from any spell list.
• When you cast a spell learnt in this way you cannot use a Spell Slot from your class(es). Instead you must expend a number of your Hit Dice equal to the level of the spell slot you want to create for the casting of this spell. 
• After the spell is cast, roll these dice to determine how much damage you will take as your lifeforce is drained by casting magic ill-suited to you. You may make a Constitution saving throw to reduce the damage you take in this way.
• Spells, Magic Items and Class features that would normally grant damage resistance, immunity or invulnerability, do not reduce the damage taken. Temprorary Hit Points are also ignored by this damage. Racial abilities however, may still apply as these are an innate part of the Spellcaster’s lifeforce (i.e. A Goliath’s “Stone’s Endurance”).
• The Hit Dice that are expended are recovered by the normal rules of resting (for your campaign). Naturally, Hit Dice expended to cast a spell in this way cannot also be used for recovering hit points during resting.