The den is located in the Ilex Forest and is home to the legendary Pokémon Entei. The stone inside the den is a key part of the story and is essential to accessing Entei.
Here's what you need to do to get Entei:
1. Find the den: It's located in the Ilex Forest, near the edge of the map.
2. Talk to the old man: He'll tell you about Entei and the stone.
3. Find the "Red Gyarados": This is a special Gyarados found in the Lake of Rage, to the west of the Ilex Forest. It's a very tough battle, but you can catch it or defeat it.
4. Return to the den: Once you've defeated or captured the Red Gyarados, return to the den and talk to the old man again. He'll then reveal the stone's secret and let you enter the den to face Entei.
The stone is a symbolic representation of Entei's power and is necessary to awaken it. It's not something you can physically remove.