If Kog'maw is a unit at a base, does his Deathknell ability (which deals 4 to all units at his battlefield) apply to that base?

Ruling: No, bases are not considered battlefields. Kog'maw's Deathknell ability only affects units at his battlefield, not units at bases.

Nuances:

• Units are at one of several locations while on the board: a battlefield or their base, which are separate zones of interaction
• If bases were considered battlefields, cards like Void Seeker could hit units at a base, which is not intended