To create a test case generator for the "Mysterious Dungeons" problem, we need to generate random dungeon grids within the specified constraints. Here's a function that generates such test cases:

```python
import random
import string

def generate_dungeon(W, H):
    # Initialize grid with walls
    dungeon = [['#'] * W for _ in range(H)]
    
    # Place the player's starting position and the exit
    free_cells = [(i, j) for i in range(1, H-1) for j in range(1, W-1)]
    start_pos = random.choice(free_cells)
    free_cells.remove(start_pos)
    exit_pos = random.choice(free_cells)
    free_cells.remove(exit_pos)
    
    dungeon[start_pos[0]][start_pos[1]] = '@'
    dungeon[exit_pos[0]][exit_pos[1]] = '<'
    
    # Place rocks and carpets
    num_labels = random.randint(1, 8)
    labels = random.sample(string.ascii_uppercase[:8], num_labels)
    
    for label in labels:
        num_rocks = random.randint(1, 3)
        rock_positions = random.sample(free_cells, num_rocks)
        for pos in rock_positions:
            dungeon[pos[0]][pos[1]] = label
            free_cells.remove(pos)
        
        carpet_pos = random.choice(free_cells)
        dungeon[carpet_pos[0]][carpet_pos[1]] = label.lower()
        free_cells.remove(carpet_pos)
    
    # Fill remaining free cells with empty spaces
    for pos in free_cells:
        dungeon[pos[0]][pos[1]] = '.'
    
    return dungeon

def dungeon_to_string(dungeon):
    return "\n".join("".join(row) for row in dungeon)

def tcgen():
    test_cases = []
    
    # Generate a random number of test cases
    num_test_cases = random.randint(1, 10)
    
    for _ in range(num_test_cases):
        W = random.randint(3, 30)
        H = random.randint(3, 30)
        
        dungeon = generate_dungeon(W, H)
        test_case = f"{W} {H}\n{dungeon_to_string(dungeon)}"
        
        test_cases.append(test_case)
    
    # Add the termination line "0 0"
    test_cases.append("0 0")
    
    return "\n".join(test_cases)

# Example usage
print(tcgen())
```

This code defines a function `tcgen` that generates a set of random test cases for the "Mysterious Dungeons" problem. Here's a breakdown of the steps involved:

1. **Grid Initialization**: The dungeon is initialized as a grid filled with walls (`#`).
2. **Start and Exit Positions**: Random positions for the start (`@`) and exit (`<`) are chosen from the list of available cells.
3. **Rocks and Carpets**: A random number of rock and carpet labels (up to 8) are chosen. Each label has a random number (1-3) of rocks placed in the grid, and one carpet associated with it.
4. **Remaining Cells**: All remaining free cells are filled with empty spaces (`.`).
5. **Test Case Generation**: A random number of test cases (up to 10) are generated, each formatted as specified in the problem statement.
6. **Termination Line**: The line "0 0" is appended to indicate the end of the input.

This generator ensures that the generated dungeons adhere to the constraints and format specified in the problem statement.