The Arena is a optional area that can be entered through a warp door. When you enter it, a popup says:
"Are you sure you want to enter? Powerful bosses await!"
If no, the game continues like normal. But if yes, you get transported into the Arena.
Bosses in the Arena
Druid Lord, A powerful druid unlike the others, and can summon 2 tanglers at once. (LVL 10+)
HP: 40 | POW: 6 | DEF: 4 | Max SP: 20
2. Maelstrom, The darkness that created all of this monstrosity. Merely with a fraction of his power. (LVL 15+)
HP: 55 | POW: 9 | DEF: ? | Max SP: 50
More bosses will await the higher level you have. EX: 3rd Arena Boss will be unlocked at level 20/25. (Note that you can also repeat arena bosses 3 times a day, to prevent EX Stone farming.)
100% - Coins, Tokens
50% - Gears, Cards
25% - Magical Stone
1% - EX Stone, which boosts POW and DEF by 3 permanently.