Resonance mechanic

This is an homerule which became in in-world phenomenon. The homerule was initially for Inspiration, a mechanic from DnD 5th (2014) edition, which I liked as an idea but found to be either finicky, mercenary (i.e. role-playing not for the fun, story or artistry of it but for a 'profit' of Inspiration point), or tedious as an actual part of the game.

Under Resonance rules any character can reroll any 1d20 roll (attack, saving throw, ability check, initiative, even a damage if they happen to roll 1d20 for damage – any 1d20) any amount of times and take the best result. 

In-world by doing so the character literally breaks the world around them, initially in a subtle ways; the striking sword gains momentum to break through the armour, the unfolding hostile spell withers and dies as it falls upon a person, the stubborn lock twists to accommodate the lockpick, and so on. It is always, always possible to be better, stronger, more masterful, or plain luckier, to come out on top of any situation.

The catch is that if the character breaks reality, the reality breaks them back. Every character starts with Resonance at 0, and every such Resonance reroll adds 1 to this score. Accumulation up to about 5 points is not overly dangerous but the higher Resonance climbs the more dangerous it becomes, and getting rid of Resonance is a long process, where barely a point of Resonance 'bleeds out' during about three months of time on its own, and a downtime action (about a month of dedicated and somewhat expensive effort) is required for a resonant person to ground themselves more in a common reality, and get rid of more of accumulated trouble. Downtime actions are only about three a season between adventures, and are useful for many other big activities (often including actual level up), so constantly 'bleeding out' Resistance through them might be a long-term detriment.

But Resonance is not just a scale – it is the cracks through which Something Else is coming through,  from the inside, and becomes a part of reality, and through wide enough cracks makes the character into Something Else. It is called Resonance because it resonates with something else, outside of the world, right around the corner and under the skin, but also a million miles away until offered an open door.

At certain thresholds of accumulation (5, 10, 15, 20 – or 3, 5, 7, 11, 13, 17, 19, 20 points, if we are playing closer to Eldritch Frontier kind of place where the reality is already fragile) the character makes a test rolling 1d20, and if the roll falls under the current Resonance, the character is acquiring a permanent resonant trait – a stain, a scar, a sorrow, a shiver – which is much harder to get rid of initially and which becomes permanent soon enough if not taking care of. Just like every 1d20 roll this one can be also rerolled with Resonance but if the new score hits another threshold, it is rolled again immediately twice with the worst result in an effect. 

If the roll fails, the consequences are rolled on tables – which I mostly pilfer from Esoteric Enterprises tables such as "What Has Your Hubris Wrought?" – and which are usually tailored to a given world (as what might be a mundanity in one world could be entirely unusual in another). The general idea here that the more a character draws on Resonance and breaks reality the more they become a local equivalent of the Spook, a grotesque, spiritually and supernaturally if not physically. Not all consequences in those tables are purely negative, but none are purely positive, and the higher is the threshold involved (i.e. 17 comparatively to 7) the harsher, more troublesome and grotesque are consequences.

Even if the character is very lucky and passes every check without consequences, high Resonance accumulation is felt on instinctive level – by beings of the world as if a high irradiation that makes ones bones churn within themselves if they stand too close, and by the beings from outside of the world as if a ticking bomb just about to go haywire (because most times it is what it is).

I found that this "Make your own sword of Damocles" principle works rather well in a game; as (unless we play in a fragile reality areas of the world) it is lenient enough to give character some leeway during the adventure while not becoming immediately a problem, but is sort of self-regulating in a heavy use. As player-controlled mechanic it eliminates finicky DM decisions and mercenary role-play, and, as initially just a number that goes up, it doesn't break the flow of the game with a necessity to accumulate Inspiration point first. I also like the temptation of success that the Resonance dangles before players: as Resonance is mostly used in times of stress and desperation it is always possible to accept a bad roll, accept bad outcome, accept bad fate but it is so often also tempting to give in into Resonance just a point more and, for a moment, succeed.