While I can't think of a "secret" way of making a single traitor, how's this for a quick mock-up of a competitive AH variant. I've kept it completely free of custom cards, it just uses the standard game cards, and where noted takes into account things like Injury cards from DH etc. This is my first write-up of it, I am making it up on the spot, so anything I miss/overlook/screw up, please forgive for now. If this sounds even half appealing to anyone I will think some more on it and write up a more fanciful version. Any play-testing done or comments/critisicm would be welcome too.
The game runs as per the usual rules except in the following circumstances:
Divide the players (or investigators if playing more than one per player) by 2. Half (round up) are Heroic investigators, the other half are Cultist investigators.
Calculate the monster limit and other such things as if the number of players were equal to the number of Heroic investigators.
Draw and resolve two Mythos cards for gate placement, but ignore the special rules listed under the Mythos Phase below.
If an opponent uses an item or spell against an investigator that has an effect on monsters above and beyond combat bonuses, the target investigator may make a Will or Luck check to avoid the effect with a negative modifier equal to the opponent's Lore.
If an investigator is hit by an effect that states it would return a monster to the cup, the investigator is instead returned to their home location (or Lost in Time and Space if in an Other World) and is delayed.
Elder Sign trophies can be spent instead of gate trophies.
Heroic investigators treat all monsters normally. Cultist investigators may ignore monsters during the movement phase as if they were not on the board, but must still face monsters normally as a result of an Arkham or Other World encounter.
Cultist investigators may choose to attack monsters during the movement phase, but they must engage all monsters at their location and all such monsters are considered to have Ambush, Overwhelming 1 (or +1 to their current Overwhelming ability) and an additional -1 combat modifier.
Opposing investigators treat each other as monsters in terms of needing to be fought or evaded during movement.
To evade/attack an opposing investigator who is in the same location as you, both must roll an evade check. Whoever scores the most successes may choose to attack or evade. If a tie occurs, the investigator with the highest Sneak wins. If you choose to evade, combat is over and you can still move this phase if you have movement points remaining (if you evade an opposing investigator who finishes his movement in your location earlier in the phase, you do not have to evade them again if your movement is later in the phase).
If you choose to attack, resolve an attack as normal, with the opposing investigator's Sneak as a negative combat modifier. Damage is dealt as normal; the number of successes you roll is the damage you deal. If the opposing investigator is still conscious, they may then counterattack in the same manner.
If both investigators are still conscious after one attack each, roll another opposed evade check, and repeat the combat, except that investigators who evade on any round other than the first cannot move after a succesful evade check; they merely end the combat and do not have to engage that opponent in any way during the current movement phase.
If an investigator knocks an opponent unconscious they may select a single gate or monster trophy owned by the opposing investigator and either take it for themselves or return it to the cup/gate marker pile. If there are no trophies to return, the triumphant investigator may instead discard one item of their choice from the defeated opponent before that investigator selects items to discard or draws an Injury card.
If an encounter names an ally who is currently held by an opposing investigator, you cannot choose to skip the encounter if the option is there, and you automatically fail any skill checks listed.
If a location has an Elder Sign, a Cultist investigator may attempt to remove it by forcing a gate burst from within another world. Instead of having an encounter at that location, draw a gate marker but do not place it on the board. The Cultist investigator must succeed at a Lore or Fight check, modified by the number on the gate marker to abuse the Elder Sign's power to open a gate long enough for him to jump through. If he succeeds the check, he is drawn into the Other World indicated. Once the Cultist investigator has navigated the Other World normally (ie. would return to Arkham and receive an explored token) he returns to the location from which he left, places the gate marker at that location, recieves an explored token and takes the Elder Sign as a trophy.
When drawing a Mythos card, a location must have a Cultist investigator there and must not have an elder sign token for a gate and monster to appear. If no Cultist investigator is at that location or an elder sign is present, skip placing a gate and monster on the board and resolve the rest of the card.
Gate bursts happen as per the normal rules, but will spawn a gate even if a Cultist investigator is not present at the designated location.
Waking the Great Old One
Do not awaken the GOO if there are no gate or monster tokens left when one should be placed on the board; instead the first Cultist investigator to the left of the first player who is not cursed gains a curse.
I couldn't be bothered right now thinking up a way to create an oppositional Final Battle. As I said earlier, if people like the idea I'll work some more on it. I've already got a couple of ideas for variant rules to what I have written, but it is yet to be seen whether the above rules work or not.
Once again, if I've overlooked anything, forgive me, it is almost 1am on a long day as I finish writing this.