Eiyuu to Kenja no Tensei Kon ~Katsute no Koutekishu to Kon'yakushite Saikyou Fuufu ni Narimashita~ - Vol. 3 Ch. 14 - The girl who leapt through time

[thewookie1]

Here's my theory, Elira is the original donator of the mana to Reid when he was on his deathbed as a child however neither remember that.

Effectively Elira used her magic and found Reid was to be her soulmate but was originally going to die very young. So she used a special magic to give him a transfusion to save him however time magic has odd effects thus causing his odd magical properties and her sudden death in mid-war.