A lot of people get confused here because they don’t recall or think the pact between Eridan and Karkat was ever mentioned.
So it is one of the things in Homestuck that was never explained.
But it has been mentioned and explained.
Karkat made a pact with Eridan that is he ever really needed it, Karkat would act as a stand in Moirail for him.
That is why Karkat was so angry at Eridan when he pretty much murdered Feferi and Kanaya. [and almost killed Sollux] Because Eridan should have came to him to talk about it but instead he took his anger out on others.