Question:

Esli'an's text reads: "Action: when this unit attacks, corrupt target unit in the defending zone."

If I am attacking a Legend through the Battlefield, may I corrupt a unit in the battlefield? My gut says yes but I wanted to double check.

I ask because I also had a Sworn of Khorne in play also. His text states: "This unit cannot attack unless the defending zone has at least 1 corrupted unit."

I thought that, if I could, by corrupting his unit I can now add Sworn's 3 P to my damage assigned to his legend.

Your gut is correct. The body part that came up with the Esli'an/Sworn of Khorne combo however is wrong. You declare attackers, in the following action window Esli'an does his thing, and then it's too late to declare Sworn of Khorne.

As always, thanks, Mallumo