Leech was a character created by MysticalMist who appeared in Sonic's Splatterhouse.
Before his corruption, he originally fulfilled his role and identity as Sonic the Hedgehog- hero of Mobius, within his universe. An unknown impostor had attacked his world and within seemingly just moments, Sonic had discovered an endless pile of bodies, including his closest friends and family, such as Tails and Knuckles.
Once this entity had gotten ahold of Sonic, he was subjected to an “eternal curse”.