Opening an Ornate Chest has a 66% change to trigger a random effect out of five possibilities. The type of trap is determined randomly, but changes each time the game is reloaded. Each Ornate Chest can only trigger a trap once. After the Ruins are reset in Don't Starve Together all Ornate Chests will again have a 66% change to trigger a trap:
A Large Ornate Chest spawns when the Ancient Guardian is killed. This chest contains substantially more loot than its smaller counterpart. In , each item in the pool has a chance to appear individually. In , 3-6 of the items are guaranteed to be chosen. There will always be an Ancient Key in the Large Ornate Chest in Don't Starve Together.
The name "pandoraschest" is a reference to the Greek myth of Pandora's box, a box that contained all the evils in the world. The fact that the chest can spawn Terrorbeaks and Dangling Depth Dwellers when opened is likely a reference to this.
Example of contents found in a Large Ornate Chest.
An example of the contents in a large ornate chest in Don't Starve Together.