Arena

Arena is a game mode in which players must defeat special enemies in modified Conclave maps until a certain number of opponents has been defeated. The first team to reach a certain score wins. Tenno do not need to extract and the mission will only end with their victory or defeat.

Mechanics
Upon spawning Tenno's Companions and gears are automatically unequipped and are announced with a special modifier that will advantage the enemies for the duration of the mission. The mission is played similarly to a Team Annihilation with respawn timers and Energy, Health and Ammo restores scattered around the map, however unlike in Conclave, players' Warframes and Weapons will keep their Mods.

Grineer Rathuum
The Grineer Rathuum is a trial by combat, set on Sedna, where a squad of Tenno takes on Kela De Thaym's Executioners in a broad casted slaughter watched across the entire Grineer Empire. The Executioners will always outnumber the squad of Tenno with additional enemies joining the fight with each Tenno joining the Cell.

Trivia

 * Arena is the first and only PvE mission type set in Conclave environments.
 * Arena was firtly introduced during the event Operation: Rathuum.
 * Arena game mode has been announced to stay after the event along with the new Kela De Thaym's fight.