What did gladiators have to pledge to Cesar before the games began?

They didn't actually PLEDGE anything, but they saluted Caesar with the Words 'We who are about to die salute you!' This was because most gladiatorial contests only ended when one of the fighters was killed by the other one- the victor would then, in turn, go on to face other duels to the death in which they ran a high risk of being killed themselves. They didn't ALL die in one contest- the most successful fighters ended up beating several opponents, after which their lives would usually be spared. Also, a beaten gladiator would be shown clemency following his defeat if Caesar was feeling merciful.