What is a glorious arena?

It's a limited time event that you can only do when your alliance has the corresponding building built. When the event is startet you can summon beasts to defeat for extra rewards. YOu can access the information from the building and then press back to event. Outside of the event you can't summon any beasts there. You don't need to use diamonds or mobility to fight the beasts.

With each beast defeated starting from lv1 you unlock the next level upt to lv 150. Of yourse the beasts get stronger with every level. Every 5 Levels you can ask for help from you alliance cause these have further increased stats and if enough people defeat these, you unlock the elite boss as an extra boss and therefore extra rewards.

Depending on what server you are not all 150 levels may be available to you at once, but unlock after the countdown timer for them has passed.