Grixis

If an ability of a plane refers to "you," it's referring to whoever the plane's controller is at the time, not to the player that started the game with that plane card in their deck. Many abilities of plane cards affect all players, while many others affect only the planar controller, so read each ability carefully.

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

Grixis may cause a creature card in a graveyard to have multiple unearth abilities. Its owner may activate any one of those abilities.

Despite the appearance of the reminder text, the unearth abilities that Grixis grants are activated abilities of each individual blue, black, and/or red creature card in a graveyard. They're not activated abilities of Grixis.

A face-up plane card that's turned face down becomes a new object with no relation to its previous existence. In particular, it loses all counters it may have had.

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 ability won't trigger again. However, the replacement effect will still exile the creature if it would eventually leave the battlefield.