The players were in an ancient ruin fighting a shoggoth over an amulet. After the players were playing tug of war with it, it activated it, tearing a gate to the far realm open. The players were sucked into it but found the amulet was protecting them from the negative effects of the far realm. I intend for the players to go to the great old ones and grovel at their feet. They will likely come across a dungeon there. How should I do it? What far realm specific mechanics should I implement.
The reference I saw called it the plane of madness, where characters can be mutated and changed. I would not do this (I would assume the amulet protected tham). Instead, think of the craziest and improbable event.
How about an encounter by creatures with the same number as the party that attack once, then do nothing but run away. Your characters move on and fight and grovel or something. Later, before they leave, then they turn a corner and find the same gruesome creatures they previously came across only to realize they are their past selves. So they run away.
Or how about for the duration of their visit, each assumes a form of a random abboration?
Dragon Magazine #330, page 21, starts with a history of the far realm (hint: we have the Elves to blame) and continues by describing the cerebrotic blot (marrow and periphery) - area where the far realm touches the world. Also, the Forgotten Realms Wiki has an interesting article on the Far Realms too.
One source says travel between layers only requires thought.
Aberrations are the default inhabitants - things that should not be. Physical traits are as crazy as you like but should always be utterly alien - if it seems normal there is not enough farplane in your horror soup, for example:
Water that you ripple through,
Air that you cant breath, it breathes you in and transforms you to an aberrant form irreversibly.
A cyclical repetition of space where no matter how far you travel you return to a nodal location. (box universe)
Body Horror, where your own flesh is no longer yours to command, either twisted or inhabited by other 'lifeforms'
Mental Dismorphia where you no longer recognise yourself, have the players switch sheets and watch as they struggle to play with unfamiliar roles where a death will kill two characters one mentally the other physically.
Temporal glitching. random skipping of turns, extra turns, recording a turn then after its finished erase it completely all hitpoints restored to all monsters and players all spells just cast restored and let confusion reign. Being in two places / times at once. (actions and movement can be spent from either of your locations but for goodness sake make sure you spend your limited movement wisely to make sure both of 'you' are out of the line of death beams)
Spatial glitching. each turn reach and spell range is altered by a random multiplier for each person, sometimes greater, sometimes lesser, meaning sometimes spells and melee will fail to work because you have no ability to attack the target 'so far away' and other times you might hit with a surprise punch 90'
If the players figure out whats going on, alter mechanics immediately. Far plane, like any horror games is about atmosphere and FP atmosphere is one of unsettling dread where nothing makes sense or is quantifiable from one second to the next in the amniotic tumour of the universe best forgot. If the players ever seem as though they want to come back to the FP, again its not FP enough. It should be something that can fundamentally scar or scare the characters.