The Betterer
Anything you can do, this monster does better. This monster wants you and everyone to improve in all ways possible, and will use its considerable abilities to push everyone to achieve those improvements, whether they want to or not, and no matter what the cost. This monster is the Betterer.
The Betterer assumes the best ability scores of all creatures within 1000', plus 1. Its savings throws are all 1 point better than the best scores of all creatures within that range. The Betterer possesses a magical manifestation of the single best melee and single best ranged weapon within that range, plus 1 to hit and plus 1 to damage. It also possesses a magical manifestation of the best armour of a creature in range, plus 1 to Armour. The Betterer has 1 more Hit Die and 1 more hit point than the highest maximums of creatures within range. The Betterer assumes all magical spells and abilities of all creatures within range, but with either a +1 (for d20 rolls) or +5% (for percentile rolls) in their favour to successfully achieve the desired or a beneficial result. Each turn, the it has one more action or attack than the creature within range who would otherwise have the highest number of actions that turn. It has specialist skills and other abilities one point (in d6) better than all creatures within range. The Betterer can instantly call forth as a free action a magical manifestation of any mundane items possessed or carried by creatures within range. The items appear as slightly better versions of whatever is present. Items can include includes mounts, transport, siege weapons, documents including magic scrolls and similar.
The Betterer appears as a combination of the most beautiful features, body parts and clothing of creatures within range. The overall effect is very attractive, but just not quite “normal”. The Betterer is extremely touchy about references to its appearance, and will violently confront those that offend it. It is open to flattery, at least in the short term, but will quickly see through any attempts at trickery, due to its high intelligence. It takes delight in demonstrating and explaining at length how much better it is than all those present in all possible aspects.
The Betterer also looks to continuous improve those around it, and consequently itself. It is often encountered overseeing extensive and brutal regimes of physical and mental trials and training, and may be overseeing a dungeon specially built or modified for that purpose.
If pressed on its origins the Betterer will most likely expound at length as to its antiquity and originality, claiming it is a true elder god, or perhaps even the sole prime being and creator of all things. In fact it is far more likely that the Betterer was once simply a mediocre and vain mortal who luckily managed to complete a deal with a more powerful supernatural being.
Armour (special: best score within 1000' range +1), Move (special: best score within 1000' range +10%), Hit Dice (special: best score within 1000' range +1), hp (special: best score within 1000' range +1), attack method (special: best melee and ranged attack within 1000' range, +1 to hit and +1 damage), Morale (special: best score within 1000' range +1). Special abilities: all spells and abilities within 1000' range, with a +1 in d20 / +1 in d6 / +5% better.