In general, d20 games are more fun if the foes have abilities that require PCs to make interesting decisions.
Ideally these abilities can be easily figured out (perhaps after being experienced a time or two), follow an internal logic, and force the players to try new things without being frustrating or overpowered.
Phase venom. A creature with phase venom is out of phase with all standard planes of existence. It takes only 50% of the damage inflicted on it, and it only 50% likely to be effected by nondamaging effects.
All the creature’s attacks infect targets injured with phase venom, causing them to be more in-phase with it, and less with the normal universe. Such targets do full damage to the phase venom creature and have nondamaging effects affect it, normally, but receive 50% less healing from allies not at the same phase, and each round are 50% less likely in that round to be affected by non-damage based abilities (such as beneficial spells) cast by allies not at the same phase. They also take only 50% damage from creatures not out-of-phase, and are only 50% likely to be affected by such foe’s nondamaging effects.
A target of phase venom becomes fully in-phase with their normal reality after one minute.
Now, this makes a creature very resistant to PC attacks, but it also gives them a way to make it less resistant, at the cost of potentially being more cut off from ally support. OTOH, if the phase venom creature is used in a fight with creatures that don’t have that ability, a PC that becomes out-of-phase is actually harder for some foes to hurt… which may cause them to target in-phase foes.
None of this is overpowering, but it adds a new element to an encounter, forcing PCs to decide who is best to face off against each kind of foe.
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A template for 1st ed Pathfinder, to turn a foe into a Boss Monster.
Initiative: If a boss’s total initiative is less than 15, it changes to 15. If that would cause it to go last in the first full round of combat, it’s initiative improves until it goes next-to-last.
HP: Double total HP
Boss Action: At the end of each round, the Boss gains an additional full-round action. It cannot use this to run, charge, or double-move if it has already done any of those things this turn.
Boss Bash: As a full-round action, a Boss can move itself up to its move and damage all creatures adjacent to it at any point as if it had hit them with a melee attack. If the boss had some kind of limitations on its movement they do not affect this action, but are still in place after it takes this action. This movement does not provoke an attack of opportunity. Alternatively, it can expend a spell or spell-like ability to damage these creatures equal to 1d6 per spell level +1d6 per 2 caster levels.
This ability looks and is described differently based on what the boss is, and does damage of a type the boss can normally do.
Boss Options: Anything other than boss bash that the boss can do a limited number of times per day, or per minute, or per round, it can do twice as often.
Boss Resilience: If a failed saving throw would normally cause a boss to be helpless, unconscious, dead, or paralyzed, and the boss still has the boss action ability, the failed saving throw instead just strips the boss of its boss action for 1d4+6 rounds.
Treasure: Give double treasure.
There you go! All the boss’s numbers and abilities are in the range PCs can deal with, but it’s twice as tough and dangerous, and harder to pin down or isolate!
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