Monsters make Save Rolls when affected by spells, special abilities, or other effects that allow a save.
Base Target Number (TN) a monster has to Save Roll equal to or higher is 16, then subtract -1 from 16 for every 20 HP it has.
Any monster with 220+ HP has a base Save Roll TN of 5. However, the caster's level increases this TN by +1 per level, making it progressively harder for the monster to successfully save.
Save Target Number*
Monster Save Target Number = 16 - (Monster HP ÷ 20, rounded down), minimum 5 Note: Monster Save TN is based on max HP and doesn't change.
Examples:
Ghoul Gnawer (22 HP): 22/20 = 1 (rounded down), so 16 - 1 = Save TN 15 Spider Empress (400 HP): 400/20 = 20, so 16 - 20 = -4, but all monster save rolls TN never lower than 5.
Save Roll is just like characters making Save Rolls, just that monsters TN is based on Save Target Number.
Roll d20 + Save Bonus ≥ Save Target Number*
Example: a monster with TN 5 dodging an 8th level Wizard spell will need to roll 13 or greater on 1d20 because TN 5 + 8 = 13 new Target Number
Open Dungeons provides a calculator for you: Monster Save Roll Calculator
