An alliance formed outside traditional structures: between rivals, criminals, enemies, or those society rejects.
Here’s a draft text for a “Bastard Bonds Guide” — whether you mean a game guide for the indie RPG Bastard Bonds , or a creative/metaphorical guide to “unconventional, illegitimate, or unlikely alliances.” Bastard Bonds – Quickstart Guide
Some bonds aren’t born of love or law — but of need, circumstance, or shared sin.