"""Decode a base-n encoded string back to an integer.

This function decodes a string that was encoded in a specified base (n) back to its integer representation. It supports custom character tables for encoding and handles negative values. The base must be between 2 and the length of the character table. Invalid characters or digits for the specified base will raise a ValueError.

Args:
    encoded (str): The base-n encoded string to decode.
    n (int): The base used for encoding. Must be between 2 and the length of the character table.
    table (Optional[str]): Custom character table used for encoding. Defaults to digits (0-9) followed by lowercase letters (a-z).

Returns:
    int: The decoded integer.

Raises:
    ValueError: If the base is not within the valid range, or if the encoded string contains invalid characters or digits for the specified base.

Examples:
    >>> decode_base_n('ff', 16)
    255
    >>> decode_base_n('16', 36)
    42
"""