Holy hell, this is really bad for Thedas. Dark Elves are no joke.
It's likely the Dark Elves would immediately find out something is wrong due to the Winds of Magic becoming something else(The Fade) the mages would no longer be able to draw upon Dhar and would instead have to make due with the Fade instead. They'd also experiment with it and twist the spirits in the fade into demons or even experiment with demons themselves and they'd quickly learn about Blood Magic which with their prior experience they'd likely excel at it and develop to further heights than Tevinter would dream of. They'd also really be curious about Darkspawn and would probably want to experiment with the tainted blood Darkspawn have.
The only reaction the Elves of Thedas would have of them would be very negative because their actions would make humans hate elves even more. Any Elf caught by Dark Elves would be regarded as a novelty but when they find out that Thedas Elves only have the same lifespan as humans they'd be thought of as lesser elves are maybe even not regarded as elves at all but pointy eared humans. There'd also be some pogroms once word of the atrocities the dark elves are doing reaches the ears of people and the elves in alienages would be having a real bad time.
Tevinter and Qunari are busy with either fighting each other or preparing to fight with each other so they'd only be able to send minimal resources to investigate the Dark Elves, Tevinter would likely send some Magister and the Qunari would assign a few Ben-Hassrath to investigate their appearance and actions. Both them would note them as a serious threat with Tevinter hating them for being a faction of elves and the Qunari hating them for using magic and mages.
If this was earlier in the timeline, the Chantry might've tried to call for a Exalted March but in Dragon Age Inquisition Chantry leadership is decimated and what remains is very fractured. The Inquisition themselves wouldn't even be able to respond fast enough to act against the Dark Elves because they'd be raiding by sea and the Inquisition doesn't have any substantial Naval assets. This actually might seem like vacation for the Dark Elves in comparison to if they were going to invade Ulthuan.
The Dark Elves might set up a new kingdom for themselves but it'd be far away from mainland and further south so they'd be furthest from their greatest threats Tevinter and the Qunari in this new world. They'd also probably like Qunari as slaves because of their uniqueness and being a new race rather than being humans or elves as they'd be used to.
Overall, I'd say you just gave Thedas a small slice of the Grimdark pie that Warhammer Fantasy is force fed on a constant basis. It'd only be worse if even a single of those ship had any Ork slaves because than Thedas will be plagued with a Green Menace within a few weeks of one of those Orks being used as cannon fodder in a slave raid with their spores going everywhere. Once one of those Ork spores lands somewhere, Thedas would have threat greater than even Darkspawn because at least Darkspawn stay in the Deep Roads and only emerge to the surface every few centuries rather than the daily threat of Orks which are often bigger and stronger than humans and live and breathe just to fight.
