[quote=@Nieszka]
That idea is a cool one to me. How would the bargain affect everyday users of soulsteel? Is there a price for using the superior weapons?
[/quote]

Use would progressively open users to possession. The cult has elite fanatics who are basically controlled by their weapons. Saliszi nobles only wear their weapons in battle to avoid such a fate. Only extremely strong willed or otherwise protected individuals could resist the demonic influence over time.