Elven High Magic

Elven High Magic is the pinnacle of the arcane Art practiced by mortals.  The precepts of High Magic are a closely guarded secret by the elves (and practically are only achievable by them due to their long lifespans).  By casting spells in concert, those trained in High Magic can achieve feats of magic that blanket entire cities, permanently alter the landscape, and work other wonders impossible without the cooperation of multiple practitioners.  Training in High Magic allows the spellcaster unique insight into the workings of magic, and makes it easier to perceive workings of magic and detect illusions. 

Restriction: Elves and half-elves only.

Prerequisites: Cooperative Caster feat; ability to cast 9th level spells

• Your primary spellcasting ability (Intelligence for wizards, Charisma for sorcerers) increases to 24, if it is not already. 
• Using this feat, you may use spell slots from others with the Cooperative Caster feat to increase spell levels beyond 9th level.  (For example, you could cast Magic Missile as a 9th level yourself, and then add 3 spell slots from one or more other casters with the Cooperative Caster feat to create 12 darts of force, each dealing 1d4+1 force damage per dart; where without this feat, even with other spell slots via Cooperative Caster, 9th level/9 darts would be the maximum.)
• You may cast Detect Magic at will, without using a spell slot.
• You have advantage on Intelligence (Investigation) checks to perceive illusions.