“An entity known only as the Serpent speaks directly to me. Others - daring to call themselves wizards, magicians, and sorcerers - manipulate the tiniest aspects of the Serpent and call it magic. But I speak to the Serpent, and the Serpent speaks back.” - Vecna
The serpent is a gargantuan creature the players saw in Vecna’s memories. They were the one who made Vecna into a Lich using a ritual.
The players theorised that the Serpent was an “Outer god” but it was impossible to tell what it truly is.