Sedris, the Traitor King

At the beginning of the end step, a creature returned to the battlefield with unearth is exiled. This is a delayed triggered ability, and it can be countered by effects such as Stifle or Voidslime that counter triggered abilities. If the ability is countered, the creature will stay on the battlefield and the delayed trigger won't trigger again. However, the replacement effect will still exile the creature when it eventually leaves the battlefield.

Despite the appearance of the reminder text, the unearth abilities that Sedris grants are activated abilities of each individual creature card in your graveyard. They're not activated abilities of Sedris.

Activating a creature card's unearth ability isn't the same as casting the creature card. The unearth ability is put on the stack, but the creature card is not. Spells and abilities that interact with activated abilities (such as Stifle) will interact with unearth, but spells and abilities that interact with spells (such as Remove Soul) will not.

If you activate a card's unearth ability but that card is removed from your graveyard before the ability resolves, that unearth ability will resolve and do nothing.