Soulmark

“Unfortunately, it's not defective. Though most people's soulmarks only recognize one other soul, yours seems to recognize two.”

Source: wiktionary

The changes to Darcy's soulmark correspond with her soulmate's state of mind.

Source: wiktionary

In the fan fiction adaptation, Someone_aka_me uses the same lines to forge a connection between Alec and Magnus, both figuratively (putting both characters in the same scene and having them react to and impact each other) and literally (this particular piece of fan fiction introduces an alternative plotline in which Magnus’s words activate a soulbond between them, as they match a pre-existing soulmark imprinted on Alec’s skin).

Source: wiktionary