Once a map is reloaded, all players within a server will be taken to a bone-shaped platform in what is seemingly an endless void. The Void Boss will emerge and unveil itself from the darkness and appears to be ready to kill the players, however, this is later revealed to be a fakeout after 5 seconds have passed. sans will reveal himself, standing on top of a bone and will chat with the players. Players will be taken to the Chungment map to further their conversations with sans, as he rambles on. After a bit of dialogue, he will ask the server if Big Chungus is funny.
Players will have two choices to make in this scenario. The server can collectively agree as a group that Big Chungus is funny by saying "Yes" or saying nothing at all to take on a neutral stance, both options leading to sans sparing everyone and will give the "You Aren't Cringe!" badge. Or, players can choose to not cooperate and exclaim "No" in the chat, resulting in sans being irritated and the fight is initiated.
There are two phases while a server is up against sans. The first half of the fight will have players facing various attacks that sans is capable of unleashing on the players. Players will be barraged by bones, incinerated by Gaster Blasters, and be cringed to death by sans while suffering under the 'Cringe' status effect.
If the players managed to survive the first half of the fight, everyone who died in the first half will be able to participate again at the checkpoint. However, the fight will become increasingly difficult as players will need to be prepared for the unique mechanics of the attacks he used in the second phase. sans will refer these attacks as 'special attacks', indicating that they possess a unique property that players should recognize if they're familiar with the Undertale series. Attacks colored in Blue indicates that you must be patient, in other words, to stay idle. By not moving, the player will not suffer from any damage for that specific attack. Attacks colored in Red indicates that you must move through it. Players moving through red attacks will not suffer from any damage, however, if they stand still while they are hit by the red attacks, they will suffer from a bit of damage. This is also held true with the blue attacks if the players performs the opposite of what is expected.
After some red and blue attacks, players are expected to survive a trial where they must reach a floating platform in time before they are killed by the Bonezone, which will periodically flash every 1.5 seconds and kill any players that are not in the platform. Players who are in the platform still need to be wary about the Gaster Blasters that is summoned, as they can still be killed while they're on the platform if they are not caught off-guard. Any players that manage to survive the entirety of the fight will be able to 'fight' sans, with a bombardment of rotating Mercy/Attack text bars scattered across the map while sans is worn out by the fight. Players must identify which of the many text bars on the map is the actual 'Fight' ability, as players who recklessly collect too many of the fake text bars being eradicated by a Gaster Blaster. Once the players find the real one, the text bar will shoot into sans. sans will state that it only feels like 0.5 damage, before being knocked out. Players who successfully defeat sans will be awarded the 'Defeat Void Boss' badge and an event item.
sans has various abilities he can use to kill the players. His move-sets include the following:
- STATUS EFFECT - The 'Cringe' Status effect is given to all players once the fight is initiated by sans, with all players losing their health over time, putting them at high-risk of being killed earlier on in the fight.
- ATTACK 1 - sans will initiate the fight by unleashing two large Gaster Blasters firing at a random player in the arena, before unleashing a frenzy of small Gaster Blasters fired in random directions at random players.
- ATTACK 2 - The arena will be swept by several elongated bones spinning through the arena, killing anyone who comes in contact with it. In addition, several, small Gaster Blasters will continue to fire at random players.
- ATTACK 3 - Three massive Gaster Blasters will be summoned at the end of the arena. The first will begin on the left side, the second begins on the right side, and the last will begin at the center. Any players caught within the blast radius will be eliminated.
- ATTACK 4 - Similar to Attack 1, players will face another frenzy of small Gaster Blasters being fired at random players in random directions. This is later followed by several Gaster Blasters firing downwards at random spots in the arena and bones being tossed at any players within close proximity of sans.
- ATTACK 5 - sans sends a row of Gaster Blasters at the end of the arena. The Gaster Blasters will fire after some time have passed, dealing damage to anyone who was unable to move to the 'Safe Zone' (a small gap in-between the Gaster Blasters that's not being targeted by the blasters).
- ATTACK 6 - sans will fire a barrage of bones at random players, with the arena being 'flooded' by a pile of bones. Being hit by the bones can either result in the players taking damage or being flung around the map.
- ATTACK 7 - sans will fire a barrage of bones at random players (similar to Attack 6), however, some bones are colored red while others are colored blue. Blue bones indicates that the players should stay still while sans is barraging the arena with blue bones while red bones indicates that the players should move while he is barraging the arena with red bones.
- ATTACK 8 - sans will cast a series of moving walls from one end to the arena to the other. Blue walls indicates that the players should stay still and red walls indicates that the players should move through it to not take in damage. Players doing the opposite of what is expected will result in player taking in damage.
- ATTACK 9 - sans will transform the arena into a 'Bonezone', with the arena floor flashing every 1 second before a layer of bones protrudes from the ground. A floating platform is available for players to stand on top of in order to avoid the Bonezone, however, a few Gaster Blasters will be summoned at the platform that will fire at the players in a specific direction (cross, random, etc.)