The Alliance rune allows Shadowhunters and Downworlders to safely share each other's abilities, giving the Nephilim access to the strength, speed, and magical abilities of their Downworlder partners, and giving the Downworlders access to the warrior skills of their Nephilim allies. The Mark itself is actually a matching pair of runes, each one meant to be drawn on each other by the partners intending to be bound. The runes are interchangeable between the Shadowhunter and Downworlder, and like most runes, it fades after use.