I've rarely seen an RPG break the 4th wall, but Runescape is a game that will happily do so. Merlin's Crystal is a quest found in Camelot, started by speaking with none other than King Arthur himself.
It's a relatively short quest, involving Merlin being trapped in a crystal. In order to free him I had to find the legendary sword Excalibur and learn the secret spell needed for it to shatter the crystal.
This quest brought in other characters from Arthurian lore, namely the Lady of the Lake and Morgan Le Faye, who were in possession of the sword and spell respectively. It was not a particularly difficult quest, with the only puzzle being about infiltrating Mordred's fortress.
The spell summoned the demon Thrantax, who enabled Excalibur to break the crystal. I had some trouble figuring out how to use the spell as I had forgotten what Morgan told me about it; the bat bones had to be dropped, not buried.
It's only the first in a series of quests related to Camelot, and a good time off all that RPG seriousness we usually encounter in modern RPGs.