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.
Existing long before the Nephilim, Clary Fray received an image of the rune through visions sent to her by Ithuriel, which she then later relayed to the other Shadowhunters and Downworlders. This rune helped them considerably in the battle against Valentine Morgenstern and his army of demons and helped the Nephilim and Downworlders turn the tide in their favor just before the Mortal War, which they ultimately won.
These are all the known alliances made with the rune:
- Alec Lightwood and Magnus Bane
- Jocelyn Fray and Luke Garroway
- Maryse Lightwood and a female faerie
- Patrick Penhallow and a warlock
- This is the only known rune other than the Mark of Cain that can be applied on Downworlders without the fatal consequences that usually result when Downworlders are Marked with angelic runes.
- This rune was mistakenly included in the published Shadowhunter's Codex.
- The artist Valerie Freire based the rune's design on the practice of handfasting.
- The rune was first created for the City of Bones graphic novel but was first shown "in action" as the Alliance rune on the cover of Shadowhunters and Downworlders.