Two thousand years ago, the three suns of the Centauri system were locked in a struggle for the control of their world, Domhan; in the end, one was killed, one redeemed, and one sealed away forever. Two thousand years later, in the aftermath of Rainbow Dash's disastrous birthday party, a strange celestial visitor has come with some shocking news for Twilight Sparkle - she is the reincarnated soul of the star Alpha Centauri...and she's coming back immediately to resume her duties as co-Queen.

Rainbow Dash, struggling with the resurrected phantom of one of her past lives inadvertently released by the visitor, is nonetheless determined to rescue her friend, no matter the cost to herself - but what chance does one pony have against the might of a star? Entangled in the plans of a dangerous third party, she chases Twilight into another world filled with kelpies and wolves and terrifying remnants of the War of Nightmares, in the belief that she needs to be rescued...and that she even wants to come home again at all.

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Approved by Twilight's Library!
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I've been told it starts off slow but gets much better as it progresses. Enjoy!