Underguards is an AU where every character "guards", or helps protect, other AUs. Error!Sans tries to destroy the Underguards to try and destroy other AUs. This doesn't have anything to link to.
There is no genocide or pacifist or game, they were made to guard AUs and keep Error!Sans from destroying them.
There are no villains in Underguards, even the evil characters are good.
All characters have the armour of Undyne and her helmet. Napstablook does not wear armour, he is a ghost and is somehow incapable of wearing armour, though he wears headphones.
Napstablook has a 9% darker shade of white than in Undertale.
Sans's bad time eye is now glowing grey instead of blue and yellow.
Temmie's hair is darker.
Muffet now has grey bows.
Asgore, Monster Kid, and Asriel's horns are made of steel and silver. Asgore now has a grey pitchfork, instead of a trident.
Flowey's colors are now a darker shade of yellow and white.
Mettaton, Mettaton Neo, and Mettaton Ex have armor over their robot bodies so they have more protection.
Undyne's armour has spikes on it and her spears are now grey.