I want to have a follower of Tzeentch quietly infiltrate the groups trust but I'm having problems with the cross purposes of Tzeentch. To infiltrate and manipulate from within they would need to fit in, so mutations and marks of chaos would give them away. However, the lore books state that Tzeentch generously rewards his followers with mutations, as would the corruption gradually do anyways. So how does Tzeentch keep his spies and puppetmasters from getting mutations of just becoming chaos spawns?

Sure, the Changer of Ways is particularly fond of blessing his followers with mutations, but you must also keep in mind that Tzeentch is a scheeming bastard and thus always has a plan. It would only make sense that when he does so, he does it in accordance with said plan (or plans, for he has many indeed), warping and molding his more or less witting pawns to fulfill whatever purpose he has in mind for them. If his current plan entails infiltrating a certain group of adventurers, whomever is made to carry out that task is given a fitting set of mutations, or perhaps none at all. Besides, such mutations can be extremely subtle, or even not detecable "on the outside" - new or improved inner organs, a more durable skeleton, better muscle fibres or what not.

Also, Tzeentch is a master of magic, deception and the terrifying combination of the two. Perhaps the character's appearance is simply an illusion, using daemonc conjuration to conceal his true form?

Yes, I give mutants living among folk more subtle mutations such as Witch Sight, unnatural age etc.

For a better idea on the range of mutations you can have, have a look at the 2nd edition Tome of Corruption.

Should the rule about mutations > toughness means they become a chaos spawn still occur? In which case Tzeentch would just refrain from giving them that last terminal mutation. Or Since they are favored by Tzeentch, he might be able to intervene?

And finally...... is there a phoenetic spelling of Tzeentch?

And finally...... is there a phoenetic spelling of Tzeentch?

As befitting the changer of ways, correct phonetic spelling and pronunciation is constantly changing. If you happen to "get it right", you will likely suffer corruption as well potentially summon a Daemon.

You have been warned.

On how to pronounce Tzeentch, I believe it is pronounced sort of like a "stifled sneeze", with the tz being like the ts in ants.

Regarding number of mutations, if it's a matter of a favored servant, I would just forget about that rule. To me, the purpose of the rule is to give the PCs a limit and a clear way of knowing the danger they are diving into. Also, consider the Mark of Tzeentch.

About the matter of the presence of mutations, surely a servant that develops obvious changes is going to be directed to plots that their appearance wouldn't interfere - and that's the kind of actions Tzeentch has made them to be part. But Tzeentch could also be testing his servant. You can never know with a Chaos God so complex. So, in the end, I would choose the way to deal with the matter based on the fluff and spirit of the adventure I'm creating. Maybe subtle mutations would be better - what that creates in terms of the chance of PCs discovering them?, what they would need to see to notice them?, etc. Maybe a spell disguise would be richer - is there a wizard PC that could eventually Magically See that something is wrong with the NPC? could it be possible that the magical disguise sometimes gives eerie feelings to those around the NPC - and what would be the nature of this times? If the NPC was in a certain mood - like exploding rage - would it be possible for the disguise to shimmer just for a second? Mold the reasons to you benefit, leave some doors opened to improvisation or better options when the game is rolling, and enjoy all of it.

Tell us how it worked out!

You want to be really devious? Don't worry about the mutations being ubiquitous. Have "fate" intervene. Just before an agent of Tzeentch is about to be discovered...have something happen (Another agent of chaos is discovered, an assassin makes an attempt on the spy, there's an attack, a bright and shiny artifact falls into the laps of the PCs, an earthquake happens, the building collapses, etc).

yeah, it might be a bit heavy-handed, but considering the whole concept of plans-within-plans-within-wheels-within-wheels-wrapped in tasty chaos pastry it might be something worth trying out.

Mutations don't always have to be outward. 50% of the time they can be changes in behavior.

I think the point of that is the "tell" of each servant of chaos. I always liked the horror movies where the daemon hid inside the person and then came out like a giant maw in the belly of some dude's girlfriend at an inopportune moment...

Khorne - bloodthirsty

Tzeentch - change

Nurgle - disease

Slaanesh - really likes tentacles

jh