HP Scaling is a property that many monsters possess, which was designed to ensure that fights take roughly the same time no matter the group size - mitigating/preventing steamrolling while still allowing for smaller groups to defeat them. If an enemy has HP Scaling, its HP will increase by a certain amount that increases with how many players are fighting it. HP Scaling will only start for additional players beyond the first one.
Different enemies will activate their scaling at different times in their lifetimes, sometimes upon spawning and sometimes after a delay. Enemies with HP Scaling will perform a player check roughly every 20 seconds, and readjust their scaling if needed. Readjustments in scaling will NOT affect the current percentage of health an enemy has. For example, a theoretical enemy has 100,000 base HP and 20% HP scaling (more details below). The enemy is weakened to 50% of its HP and there are 9 players in the area, so it has 130,000 out of 260,000 HP. Three players join (so there are now 12 players) and the enemy takes no more damage until it readjusts its HP scaling. After readjustment, it will now have 160,000 HP out of a total of 320,000 HP, which is still 50%.
Currently, two types of scaling exist: linear and adaptive.
This is the scaling used by many bosses within the Realm itself. For every player beyond the first, a scaling enemy will increase its HP by a fixed percent of the original, increasing its maximum HP at a linear rate.
All regular enemies in the Realm scale at +10% Base HP per additional player in Realm, while some special enemies scale at 20% per player instead. The linear scaling can be summarized in the following formula:
Scaled HP = b(1 - s + s*p)
where b is the base HP, s is the scaling factor (either 0.1 or 0.2) and p is the number of players in the realm. A Realm enemy will gain a theoretical maximum HP of 940% of the base value at 85 players, or 1,780% if it had 20% scaling.
All bosses and many enemies found in dungeons use a modified scaling system known as Adaptive Scaling, which uses a quadratic model rather than a linear one. Enemies with this type of scaling will gain comparatively less HP for the first couple of players, but the amount added per player will gradually increase as more players fight said enemy, causing the enemy’s HP to increase at an increasing (quadratic) rate. This means that small groups will not have to contend with scaling as severe as the usual model, but larger groups may have to deal with far higher HP values than linear scaling gave. The first enemies to use Adaptive Scaling were those in Oryx’s Sanctuary, but as of Exalt Version 18.104.22.168 (Sep 2020), it is used by all dungeon bosses and many dungeon enemies.
The observed formula is as follows:
Scaled HP = b * (1 + P[p-1] + A * P * [p-1]^2)
where b is the base HP, and p is the number of players in the dungeon. P represents a “per player” variable, which will be set to 0.1 for dungeon bosses or boss minions exclusively featured in boss fights, or 0.05 for all regular dungeon monsters. A represents an “adaptive scaling” variable, that for now is always set to 0.08. Therefore, the true formulae are as follows:
For Dungeon Monsters: Scaled HP = b * (1 + .05 * [p-1] + 0.05 * 0.08 * [p-1]^2)
For Bosses/Boss Minions: Scaled HP = b * (1 + .1 * [p-1] + 0.1 * 0.08 * [p-1]^2)
This, in turn, gives monsters a max HP of 2,058% (3,342%) of the base value at 65 (85) players, and bosses and their minions a max HP of 4,017% (6,585%) of the base value.