D20 Mechanic: Potency
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Potency is a concept that is not, at its core, unique to Trinity, but this particular application of the concept is, so far as the d20 System as a whole is concerned. Ironically, this concept is heavily inspired by D&D 4th Edition.
Potency was born out of the desire to standardize the caller's eikons in order to reduce player workload in understanding how each summon worked. By invoking a common table of values, it was believed that we could reduce lookup times and make it easier for players to understand how the eikons compared to each other in terms of combat value. However, since that initial inception, it has been found that the potency concept is also incredibly helpful in encounter design and class balance.
Potency is currently a work-in-progress. Moving Trinity into a potency-based system is yet another departure from core d20, but one that will undoubtedly be significantly rewarding in terms of easing play and making class balance that much easier.
Level-based scalars in spell design in d20 was a solid concept, and allowed for casters to become more effective as they gained levels. While this paradigm works in a "linear warriors quadratic wizards" setup, with the introduction of maneuvers to Trinity and the massive class rebalancing (which is currently still in progress as of 15-10-21), level-based scalars are no longer sensible.
Of primary concern is the fact that level-based scalars introduce planned obsolescence into spell design: while magic missile retains some punch at higher levels, the fact is that it is simply an ineffective spell once characters enter high level play. Much of Trinity caster redesign has focused on reducing the total number of spell levels for the majority of casting classes. As such, allowing lower-tier spells to retain their power at higher levels is a solid way of allowing casters to function for extended periods of time, alleviating the "15 minute workday" problem.
Potency is the means by which this is accomplished. Rather than fixed damage codes for individual abilities, any ability that has a numerical value attached - primarily damage and healing, but not limited to these - instead has a potency, which is a damage code that scales with character level. A spell may say that it deals potency 4 fire damage; for a 1st-level character, that spell deals 1d2+9 damage, while for a 20th, it deals 2d8+55 damage. The relative effectiveness of the spell remains the same over the character's lifetime.
This also resolves issues of multi-classing at higher levels being ineffective. A potency 4 effect is always a potency 4 effect; if a 19th-level einhander decides to take a level of mage, the spells they can access will be just as effective as those of a character who has been a mage all their life. The distinction is that the 20th-level mage will be able to cast those spells effectively all day, while the 19th-level einhander/1st-level mage will have only a few MP, and must be significantly more sparing. Likewise, the mage will have access to effects that can hit multiple targets, have longer durations, or have higher potency.
The table of potency codes is listed at the top of this page for ease of reference in play.
Potency and Size
Potency is modified to account for differences in size. This has two effects.
When a creature makes a targeted attack, the potency of that attack is reduced by 1 for each size category the target is larger than the attacker, or increased by 1 for each size category the target is smaller than the attacker. If the potency would be reduced to 0, it is instead reduced to half 1 potency; if it would be reduced below 0, the attack deals no damage. This effect does not allow you to break the damage cap.
When a creature makes an area attack, if any creature in the area has a space that is larger than the effect's radius, or half its length if a cone, reduces the potency damage it takes by 1 for each size category difference. If the potency would be reduced to 0, it is instead reduced to half 1 potency; if it would be reduced below 0, the attack deals no damage.
Half and Double
Some effects will call for half potency, while others will call for double potency.
In the event of a half descriptor, roll the indicated potency normally, then halve the result, rounding down. In general, higher-potency effects will not use half, but will instead simply use a smaller potency value.
In the event of a double descriptor, roll the indicated potency normally, then double the result. In general, lower-potency effects will not use double, but will instead simply use a larger potency value.
June 18, 2016: Complete revamp of potency calculations. Potencies have sensible average values, and the deviation from that average is roughly 10% for a given potency. Thus, any given potency roll should fall within 10% of its expected value; this feels like sufficient variation to not make things too predictable, while maintaining the utility of potency (which giant dice ranges fail to actually achieve).
Feb 6, 2017: Yet another revamp of potency calculations. This is due to my loss of the calculations when my laptop got smashed, so I had to redo them from the ground up. Earlier I had made note that some of the potency values seemed a bit off, and were ramping faster than expected; this has been resolved in the new values. Specifically, we now have true parity between potency values and hit point calculations (ie, a potency 6 attack against a character whose class gives them 6 healing surges as a base will always take them out around 50% of the time).
Note that every 2.5 points of Constitution you have above or below 10 will increase or decrease the potency required to one-shot you by 1 in that direction. So a character with a "d6 HD" class with a 12 Con effectively has a d7 HD (with a commensurate 50% chance to be one-shotted by a 7 potency attack), while a character with an 8 Con would effectively have a d5.
At even levels, due to potency only scaling at odds, your chances of surviving an equal-potency attack increase, usually significantly at lower levels and not significantly at higher levels. I may need to do some math on this, it's possible we may need to move to a "potency scales at every level" paradigm.