as far as we know, eccentric party night is not real! unless they performed it at OO which we'd have no way of telling (lol) i believe it exists solely for the fans and exists solely as what could have been. i think people have theorized it might have been written/composed in natsume's original proposal, but i believe all that's been attested to have been inside it is just the plan for fine and the eccentrics to face off and turn the tides.

from what i remember, the original mix was made from a "reminiscence" perspective, as a spiritual nod (and gift to the fans) to the existence of the five eccentrics back then. that is why rei sings in his usual tone and not his 2nd year tone!

the reminiscence selection mix was remade for the element anime, as the MV accompanying it (while not real) would portray them in their 2nd years. special thanks to rei's VA (toshiki masuda) for insisting on the vocals being re-recorded