The Coliseum Ruins is a location found in Breath of the Wild.
The Coliseum Ruins were once an area used for combat, but like much of Hyrule, they have fallen into despair. They are now filled with strong monsters. There are numerous Black Bokoblin, Black Lizalfos, Black Moblin, and a Lynel that infest the area.
Just outside of the Coliseum Ruins to the south, near the Aquame Bridge, a Hylian named Dai can be found. He wants to enter the Coliseum to fight the enemies, winning their weapons and some treasure. However, he cannot quite bring himself to do it and instead just stands outside, trembling in fear.
A treasure chest can be found at the very top of the Coliseum at the far north end, which contains five Bomb Arrows. A second treasure chest can be found at the top of the ruins at the south end, with this one containing an Amber gem.
The enemies at the Coliseum Ruins will vary in which equipment they hold, depending on how many Divine Beasts Link has defeated. At the center of the Coliseum on the ground floor, Link will encounter a Lynel. On the second floor, there are three Black Bokoblin. There are three Black Lizalfos on the third floor and three Black Moblins on the fourth floor.
Zero or One Divine Beasts
If Link has not defeated a Divine Beast, or if he has only defeated one of them, all enemies in the Coliseum will carry weapons from the Soldier class.
For each of the three types of enemies on the upper floors, one of the variants will carry a Soldier's Spear. Another one of the variants will have a Soldier's Claymore. Then one of the variants will have a Soldier's Broadsword and a Soldier's Shield. So if Link defeats all three Bokoblin, he can acquire all four items in the Solder class of weapons. Defeating all the Lizalfos, will give him a second set of those same weapons.
Two Divine Beasts
After Link has defeated two Divine Beasts, all of the enemies in the Coliseum will receive an upgrade. Instead of the Soldier class, all enemies will be upgraded and will now carry items from the Knight class. This includes a Knight's Halberd, Knight's Claymore, Knight's Broadsword, and a Knight's Shield.
Three Divine Beasts
After Link has defeated three Divine Beasts, all of the enemies in the Coliseum will now carry items in the Royal class. This includes a Royal Halberd, Royal Claymore, Royal Broadsword, and a Royal Shield.
The Lynel on the main floor will be upgraded to a White-Maned Lynel, but will still carry a Savage Lynel Sword, Savage Lynel Shield, and a Savage Lynel Bow.
All Four Divine Beasts
After Link has defeated all four Divine Beasts, all of the enemies in the Coliseum will now begin to carry elemental weapons. The three Black Bokoblin will carry a Flameblade, Great Flameblade, and a Flamespear. The three Black Lizalfos will carry Frostblade, Great Frostblade, and a Frostspear. The three Black Moblins will carry a Thunderblade, Great Thunderblade, and a Thunderspear.
The Lynel on the main floor will be upgraded to a Silver Lynel, but will still carry a Savage Lynel Sword, Savage Lynel Shield, and a Savage Lynel Bow.
When playing in Master Mode, there will be Silver Bokoblin, Silver Lizalfos, and Silver Moblin as the base enemies. Furthermore, the Coliseum will begin with a Blue-Maned Lynel. After all Divine Beasts are defeated, a rare Golden Lynel will appear.
Nearby Korok Seeds
Pick up the rock.
There is a log propped up on another log. On the end that is propped up you will see an acorn in the tree. Shoot at it to reveal the Korok.
On the top floor of the ruins of the Coliseum there is a rock that you can pick up.
There is a fairy light traversing through the water. Run up to it and examine it.