I have a problem with my player's habit of breaching pacts I introduce to the game. Usual d10 of failing CP and negative disposition from psykers and daemons doesn't seem to be a big deal to him (**** dice didn't roll high enough to give him mutation), which makes them seem of no consequence.

Once he breached the pact of coperation with a daemon, and on the last session he outright betrayed the party, breaching the pact of fate between them.

What do you think are more adequate mechanical or non-mechanical consequences of such behaviour?

If the failing corruption isn't enough, have the daemon get upset and plot retaliation.

I assume that these breaches and betrayals are not part of a plan the player has discussed with you as part of their personal goals. I would contact the player separately from the rest of the group and ask if they have some sort of end goal for all of this, or if they are just being "lol, chaotic".

If they are just being randumb, then inform them that the consequences for their actions will begin to escalate as the number of enemies they make (NPC and Players) increases, and then make good on that. Have the daemon that he betrayed contact one or more of the player characters to see if they are interested in getting revenge.

By breaching a pact with a daemon, he's really playing with fire. Daemons don't like being double crossed or failed and if the player does it enough then he will be making a very powerful adversary. To be honest, corruption is nothing compared to what you could do.

What sort of daemon is he betraying may I ask? Is it a Khornate, Tzeentchian? Depending upon whom he does cross, we can help come up with suitable methods of long term retribution. Also what character class is he? What's his archetype and how is he playing his character?

By this for example, if he's angered a Tzeentch daemon and he's a stealthy character, the weavers of fate may conspire to make the light shine on him at just the wrong time to allow him to be seen and have that camera spot him when he thinks he's clear.

If he angers a servant of the Grandfather Nurgle, then foul toxins and plagues may soon find themselves befalling him. Disease and illness become more common and his weapons and armour end up becoming frail.

There are plenty of curses and rituals in Tome of Excess I think, which you could have used on him.

This usually calls for Complications, player specific Complications.

You could set up a scene where he will have to rely on the trust of the others to get by/survive.

(Fail that agi check and almost fall down into a gaping abyss, have him hang on a ledge with the party standing above, looking down?)

At one point maybe he will realize that his earlier actions won't get him many allies and withour them - you are just one lone Heretic against a galaxy that hates your guts.

Even worse, is when your fellow Heretics shun you..

In our group, betraying the rest of the group would probably end up getting your character killed. Some player conflict is encouraged, its when you piss of everyone that things get messy. Especially if said character is not a potent sorcerer or some such that could possibly take out every other player at the same time.

Tip for survival: One does not screw around with potent daemons and expect to come out on top, unless one knows said daemons True Name, in which case one owns its ass.