And then to the grind of war.
(This is familiar. She was a warrior as Turin, not a soldier - but she has put on the general's hat more than once in the many lifetimes since. She needs it, against Ares.)
(The default flow of this - two hosts. Fingon's hidden in the mountains to the west of the great ashen plains, Maedhros's openly marching from the eastern passes. The intent - to draw out Morgoth's forces, and catch them between the two hosts. It ends in disaster, always - not the worst plan if you assume your opponent is stupid enough to leave the safety of their walls, but, well. Perhaps the original Morgoth, when this plan was woven into the tapestry, was that dumb. Fate never has been. Ares as Sauron certainly isn't; a host openly marching across a barren plain to your mountain fortress is a great opportunity to test out your ranged weapons. Even if she sends her hosts out to meet the union, though - the commander of the elves of Nargothrond, among Fingon's host, breaks ranks when his captive brother is brought out and tortured, destroying the 'stay hidden' plan. Fingon's host follows them, and they make it inside the gates of Angband - where the inevitable trap closes, slaughtering many. As for Maedhros's host - betrayed by the Men under Ulfang's command, delayed, beset by a sudden rush from Angband - scattered and killed. Turgon's host had come unlooked for, and stayed out of the initial rush into slaughter, coming forth to rescue Fingon's host - and then the combined hosts were again overwhelmed, Fingon slain, Turgon in a retreat brought only by the life of nearly every member of the House of Hador - including the capture of Hurin...)
(She's pretty sure she can avoid that much disaster.)
The known hosts - the elves of Hithlum under Fingon, the elves of the Falas under Cirdan, the men and elves of Nargothrond under Finrod, the men of Brethil under Haldor, the men of Dor-lomin under Huor and Hurin, the Noldor of Eastern Beleriand under assorted children of Feanor, the men of the east under Bor, the men of the east under Ulfang, and the dwarves of Belegost under Azaghal - and, this time around, the elves of Doriath under Luthien and the scattered House of Beor gathered under herself as Beren.
They need Morgoth's hosts to take the field, even when it's very clearly a trap. Sauron - probably doesn't have full, unambiguous command, and she knows how the simulacrum will act -
So, the simulacrum needs to think she's being more clever than her enemies, and the bait needs to be shiny enough she'll risk it. Not too hard. The simulacrum is Fate's personality filtered into the shape that'll follow Morgoth's steps, and, well.
Fate is actually pretty incredibly reckless at a fundamental level. She enjoys gambling. She's arrogant, but - not actually very attached to winning any particular game without a significant stake, more than the simulacrum's life could offer. It's honestly no wonder the simulacrum is driving Ares-as-Sauron up the wall. (She learned caution eventually, but the simulacrum is far closer in lived experience to Fate-as-Turin than to Fate-as-Mygwainor.)
Karin-as-Celegorm thinks 'taunt Morgoth with the stolen Silmaril and then ride away very fast' sounds hilarious. Convincing everyone else is... A bit of an uphill battle. But she does it eventually. (Everything they can do is going to be obviously a trap, anyways, might as well get the simulacrum into a playful mood about it. She'll make more mistakes, like that.)
Setting up ambushes off of that is incredibly easy - she positions the forces better at woodscraft near the front of the pass Celegorm will be heading for. Doriath, the men loyal to herself, Haldor, and Hurin and Huor. They'll let any pursuit past them. Heavy infantry deeper in, everyone else graded between - Ulfang's very-likely-to-be-traitorous forces given a flattering but actually entirely useless position they'll have trouble striking at the other forces from...
It almost even works.
Well, they do crush the forces Morgoth sends out. They lure Glaurung over a trap that exposes his vulnerable belly - Mygwainor is more than a bit viciously satisfied with killing dragons. The battle spills out of the ambush, Ulfang turns on them, but they keep command of their forces - Turgon comes to bolster them, unlooked for, turning the tide back again -
If the forces of the Union are wiped out, they'll take the forces of Angband with them. Better than how it usually goes.
Her, Luthien, Hurin, Huor, and Haldor's positions are overrun in the first swell of chaos, the first time the order of battle breaks down. They scatter. Mygwainor regroups her men. She merges into Hurin's host, she mostly keeps in sight of Luthien's, though they have quite a few orcs between them -
(Mygwainor might have underestimated how much she's been annoying Sauron. Sending out this many of their maiar is definitely both reckless and excessive. At this rate, the two forces really will destroy themselves against each other. But it's successfully trapping her forces - her-Turin's father's forces - )
(She gets separated from Hurin and Huor, and sees, almost prophetically, the blow that will kill her uncle, whose wife is of Beren's House, who'd been so kind to young Turin - )
The charge she makes to rescue them is - reckless.
Huor falls, but he's not dead, and medical care could come - their position's been thoroughly overrun, and she should be retreating, she shouldn't be striking out like this -
Hurin has stood himself over his brother, ordered his men to retreat back to Beren's forces, his axe flashing around, slaughtering the orcs trying to press him -
Mygwainor - Fate - Turin - usually doesn't watch this part of the battle. Usually leaves the entire segment of Turin's life to the simulacrum, unless she's trying to throw this battle -
She gets to her target. She gets Huor moving toward the medics, bellows at Hurin to fall back, to return to his family, his people -
She faces the hosts of Angband with her sword in her hand and a snarl on her lips, and as a mortal she defeated Ares-as-Morgoth in single combat, once -
The tapestry doesn't really care about that, though. It cares that this battle is a loss. It cares about Hurin's capture, and Huor's death - it cares that Beren should have died years ago at the jaws of a wolf, protecting her father-in-law -
(Cousins are close enough, aren't they?)
'Oh, fuck you too, Eru,' is Fate's last thought, before Carcharoth - the greatest werewolf - tears her apart.