Darkmoon Duelist

Opening the chest takes about 10 seconds, and inside I've found:

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• 5 Darkmoon Prize tickets;
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• 94 gold;
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• [item=74034];
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• and 3 Mythical Healing Potion [these are most probably random].
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Upon looting the trinket you will get a Darkmoon Duelist achievement; the item itself has a quest Pit Fighter attached to it, which you should hand in to [npc=55402], standing outside of the pit.

Doing so will unlock another quest on him, a stationary one - Master Pit Fighter, which requires you to win the deathmatch another 11 times and collect another 11 [item=74034] trinkets; completing that will award you with [item=74035] and the achievement Darkmoon Dominator.

edit: why downrate?..

Looks to be the new version of Gurubashi Arena Master.

If it's anything like the Gurubashi Arena, you get the achievement upon looting [item=74034] from a chest spawned in the middle of the arena most likely located at the upcoming Darkmoon Faire Island in patch 4.3.

The chest spawns every 3 hours (12am, 3am, 6am, 9am, 12pm, 3pm, 6pm, 9pm). But be aware, once you enter the arena you are flagged as pvp and everyone who is not in your group can kill you (even people from the same faction), so it's a good idea to bring a group of guildies or friends to tip the odds your favor if you really want the trinket.

Most of the time won by a solo rogue joining while everyone is killing each other