The Prince of Frost
One option is the Prince of Frost, detailed in Dragon Magazine 374 and discussed here on Eberron author Keith Baker's blog. As I understand it the Dragon article was written to fit in any D&D setting, but it was written by Keith Baker with the specific themes of Eberron and Thelanis in mind.
This Fey could be a suitable, if somewhat villainous, patron for a Valenar elf. The Prince's story is that of a scorned lover - he awaits the reincarnation of the mortal he loved and the warrior his beloved spurned him for, and spends the rest of his time tormenting mortal heroes out of spite.
That fits with one angle on the Valenar - their culture is focused on emulating the ancestors through conflict, so finding mortal heroes and making life difficult for them isn't exactly out of your area of interest. It might make for some tension with the rest of the PCs, and you may have to prioritise your goals against the Prince's; depending on what exactly you and the other players want out of the game that could be a positive or a negative!
There are some alternatives that don't answer the question you asked (and which I wouldn't post without having led with a published Fey that might be suitable), but which might be useful to you.
Adapt a real-world fairy story
Keith Baker also writes here about ways to adapt real-world fairy tales as Fey in Eberron, using the example of the Sleeping Prince (based on the Sleeping Beauty story).
For a more Valenar angle on that, you could take the recurring figure of the Woodcutter or the Hunter, as seen in Red Riding Hood, Snow White, and the like - an outsider figure set apart from more civilised folk, who does the dirty work of killing that others might scorn, but who can be swayed (as in the Snow White story) by appeals to whatever kind of virtue they value, would work very well as the patron for a PC. Such a fey could easily find common ground with the Valenar.
The Hexblade patron
Some of your reasons for considering an Archfey Pact might also support the Hexblade from Xanathar's Guide to Everything - drawing power from a legendary cursed weapon, rather than a lord of the Fey, would also fit well with Valenar themes.
As a culture that focuses on the stories and deeds of heroic warrior ancestors, it makes sense that legendary weapons might come up in some of those tales. The earliest ancestors were a slave insurgency against the Giants in Xen'drik, so the flavour focus on Shadow does also fit - stealthy violence is very much a part of Valenar culture.
There's also a good hook in the Valenar section in Rising from the Last War:
The ancestors of the elves fought goblins for control of this region many thousands of years ago. Relics of this struggle are still scattered across Valenar and the Blade Desert: ruins, haunted fortresses, and battlefields that have slipped out of alignment with time.
That opens up further options; whatever cursed blade you've pacted to could be an old Tairnadal Elf relic, perhaps one lost in one of these magical anomalies, but could as easily have roots in the goblin cultures the elves fought - the Dhakaani Empire were some of Eberron's most gifted creators of magical weapons, and claiming power from a trophy claimed from a foe is, again, very much in line with the Valenar focus on warrior-heroism and their roots in a rebellion.
The other reason I mention the Hexblade is the mechanics - medium armour proficiency, shield use, and the option to use Charisma for attack and damage rolls fits nicely with the idea of your character as someone who's emulating a warrior ancestor, but using their pact magic (and high Charisma score) to make up for lacking the aptitudes their ancestor's followers would usually rely on.