Anchor of a life blood
In November 2016, an anchor of April MacLean's life blood was used by a Shadow Kin consultant, Rannus, in an attempt to force Corakinus' heart to stay in his body, rather than swap between his and April's. It appeared to be a blue light. However, the anchor failed, and an enraged Corakinus executed Rannus.
Corakinus' next consultant, Kharrus, also suggested they used the anchor. She commented that Rannus had been "too cowardly to go the extra step" and explained that they could attach Corakinus' whole body to the anchor rather than just his heart. This meant that wherever the heart went Corakinus would go with it, although there was a risk that the heart could be torn in two, killing both Corakinus and April. Believing the anchor to have failed again, Corakinus executed Kharrus, but later discovered that the experiment had succeeded, and used the anchor to find April.
After Corakinus opened a portal back to Earth, April passed along the anchor to her mother Jackie, which cured her paralysis, before both she and Ram Singh fled through the portal. The act of giving the anchor to her mother enabled April to have a connection with Earth even when at another part of the universe. (TV: Co-Owner of a Lonely Heart)
Jackie was able to feel April, and through their shared connection she was able to open another portal directly to the Underneath. Once the portal was open, April and Ram's fathers, Hew and Varun, entered to find the two teens.
Later Corakinus cut the connection between him and April. (TV: Brave-ish Heart)