Coliseum

General Info
The Coliseum is an arena introduced in OPTC 5.0 (in September'16) where you can fight and obtain characters that do not appear anywhere else in the game. Ideally, you will want to clear all smaller battles and then farm the boss posters and other rewards on the toughest, 30-stamina "Chaos" difficulty (it has ~6,300 EXP but is usually pretty tough). Now, on to details.

Coliseum appears on Extra Island on Wednesdays, Saturdays and Sundays. Every new character stays for 3 weeks, which means you can encounter them over 9 das. They go through some sort of rotation and reappear every few months when new Colosseum batches are released, similar to Fortnight release schedule.

Loot in Coliseum includes turtles, cottom candy, cola, forbidden tomes (aka socket books), boss posters and manuals for the boss characters. Non-boss characters, which the game can pull from anywhere, WILL NEVER DROP A POSTER. As usual, the drop rate increases with difficulty. Clearing each battle for the first time will generate some extra rewards, from rainbow gems to other loot.

Coliseum has 3 different tournaments for every wave, with 3 difficulty levels: The opening battles of each arena must be cleared in order. Every battle is always 5 rounds long, no exceptions. Once you clear an opening battle, the next one is unlocked. You will face the droppable boss in the last battle.
 * Exhibition (3 easy battles, 2x10 stamina and the 15 stamina boss battle)
 * Underground (4 medium battles, 3x15 stamina and the 30 stamina boss battle)
 * Chaos (5 hard battles, 4x20 stamina, and the 30 stamina boss battle. The difficulty here, particularly of the final battle, can be compared to 40-60 stamina Raid/Clash).

Note: The boss battle on Chaos always features the enemy from the 4th opening battle on the 4th round. Chaos boss battles usually work like this: you get 3 rounds of grunts, boosters and evolvers (one of which always has a turtle that hits for ~2.2k in case you need to stall), then the enemy from the 4th opening battle, then the actual boss. The enemy on the 4th round generally has considerably less HP than what they had when you fought them on their opening battle, and sometimes they hit for slightly less damage; specifics vary from sub-boss to sub-boss.

As usual, quality of characters you can recruit varies. Some characters can appear only on some difficulties, and as usual, the ones exclusive to the hardest (chaos) level are probably the most useful.