Attunement

All magic items require attunement before use (DMG 136). Attunement requires a short rest (1+ hours) exclusively focused on and in continuous contact with the item. "An item can be attuned to only one creature at a time, and a creature can be attuned to no more than three magic items at a time." (DMG 137)

However, the Law of Contagion applies to attunement with a magic item. There will always be a latent link between an item and its creator. Furthermore, once a being has attuned to an item, there will always be an additional latent link. The potential of the link to an item's owner is weakly dependent on the item's level/rarity and strongly dependent on the duration of the attunement. Hence, the strongest link is a legendary item attuned for decades or longer. An old magic item may have had many owners, hence many links.

The most obvious use of a latent link is using it to target a lethal spell. However, the link can be used for any magic, such as Clairvoyance, Detect Thoughts, or rendering the target infertile.

The latent link from attunement is only one of many possible Contagion effects. For example, even a mundane murder weapon will usually have a Contagion link with the victim.