She is a gate keeper along with her father. She likes to jump into death duels.
Couple those thoughts with the fact that bounty hunter killers - like the lady with the hat promissed MC to kill him for his sword - they know Nimue lives in Alfheimr and she makes swords with outworldly powers - even the demons knew that sword was imbued with outworldly powers - why do you think they used it to try and assassinate MedB? - you should know by now that all the realms we know of - nowhere, Alfheims, Midgard and the demons know of the powerful artefacts and of Nimue existing in Alfheimr - so we got literally countless factions and powers and bounty hunters and assassins who know how to get to the entrance and might also know either who is there - Fin - the hothead elf death dueller - or that they would welcome a battle with the elves - like the demons outright did in neverwhere.
The list of possible culprits is endless - starting with the hat lady who might have gottten trigger happy waiting on MC who kept extending his stay while she wanted to kill him and get the sword already.
The one established super power is: Nimue grabs MC from the moment in time they talk to her, or just asks Fin to come with - they all have boners bigger than MC when it comes to infatuations with Nimue - then she delivers her and MC back to Midgard, or simply gets her back to her mom a couple of days after the funeral
Nimue might not necessarily be a goddess per se - but as a nimph she has access to the power of the lake, that power is massive, like stargate levels of massive.