No, this isn't a thread for players wanting stats to play as a Shambling Mound...
It's a fairly tried-and true method for placing the DM's voice to create a dedicated follow-the-party NPC...what about asking a player to play something very specific for the sake of story? In other words, planting a PC in the party to deliberately introduce a sub-plot.
For example, a party is composed of good-aligned PC's. They have uncovered a growing cult of undead-worshippers, fueled by an unknown power. A new PC joins the group, dedicated to the discovery and destruction of said cult.
Unknown to the other PC's, this new PC is actually an evil cleric of the dominant god of death and undead, bent on eradicating his patron's new competition. Clearly, the evil PC has a reason to stick with the party, their goals being similar; however, the paladin-types in the group will have some serious misgivings about the evil character once they find out, and even the more forgiving PCs may be upset about the deception.
Out of game,the DM requested the player to play this concept, and, for the sake of reducing the hellacious metagaming some players tend toward, asked the player in question to keep this portion of character concept secret, so that the players and characters discover the truth at the same time.
I've had mixed reviews on this idea with those I've pitched it to...So, what do you think? Is this practice of "planting" a PC concept acceptable/ethical/as-unique-as-I-believe-it-to-be/etc?