The Dominion medallion is an item involved with the Dominion Tower. It is obtained by talking to the Strange face within the tower and asking for the medallion. It can only be obtained if all the requirements to start fighting in the tower are also met (level 110 combat, any 20 of the 38 quests listed here). It has a number of uses in the tower.
When worn inside the tower, the medallion will boost your combat skill levels (Attack, Strength, Defence, Ranged, and Magic) by up to 4. Outside of the tower this boost is not provided.
It can be used for teleporting to the Dominion Tower (limited to 3 charges per day). This teleports the player inside the tower next to the Strange face.
It can also be used to "tag" a player fighting in the Dominion Tower, which allows one to "spectate" the other player from one of the spectator points along the edge of the arena. This requires 'accept aid' to be turned on. It can be upgraded to a Super dominion medallion after reaching level 20 of Endurance mode. This can be further upgraded to an Extreme dominion medallion after killing 1600 bosses in Rumble mode.
Selecting Check Status on the medallion will give information on the fight currently set up (which fight mode, floor, monster) in addition to the current Dominion factor.