Event Mode was created as a permanent Mode in Beta 25.0 alongside the new Ranked System. This mode brings in special rules to normal games to bring out new mechanics, create complete mayhem and/or provide a bit of a challenge in deck making.
Season 3 (October 2016)
Frisk vs Chara
Rules: All players start in a random team. When entering a match, you will only fight someone of the other team. With each win, you will gain 1 point for your team and convert the enemy. If you lose, you lose all your points and be converted.
Reward: At the end of the event, the team with the most points win and everyone in it gain 5 Packs. Those who lost gain 2 Packs instead.
A special Gaster themed Haloween Event. Play against an opponent in a chaotic, glitch themed version of the game; hands get swapped, cards change stats, aesthetic, or even alter or disappear entirely as you try to defeat your opponent. All while under the added possibility of anyone being blasted by a Gaster Blaster at any time.
Season 4 (November 2016)
Rules: At the end of each player's turn, all monsters on the board move one space to the left or right, depending on the direction. Monsters that go over the edge will die instead.
Special card: Both players will have a special spell card in their hand that allows them to change the direction of the Treadmill with each use.
Season 16 (November 2017)
The return of the Gaster Haloween Event.
Season 24 (July 2018)
Rules: Your turn takes 10 seconds. Playing any card ends your turn.
Season 25 (August 2018)
Rules: Monsters in your deck start with 1/1.
Banned cards: Armor monsters and spells.
Season 26 (September 2018)
This Event was based in that everything in the game was randomized created by the game itself, including the Deck, SOUl and artifacts. The Contributors would be able to play with Shiny cards.
Season 27 (October 2018)
The 3rd year of Gaster.
Season 28 (November 2018)
This Event is very simple. The only rule is that no monster is able to counter-attack, meaning that direct combat will not result in DMG to the attacker.
Since Beta 30.0, this Event would simply make all monsters have Ranged.
Season 29 (December 2018)
Another Randomized Event. But this time, only your deck is randomized by being replaced by random Legendary monsters. Again, Contributors will have Shiny cards.
Season 30 (January 2019)
Same rules as last time, but instead of Legendary monsters, your deck is filled with Deltarune monsters.
Season 31 (February 2019)
Players start with 100 HP. Any card they play deal their cost as DMG to the user.
Season 32 (March 2019)
One rule: Turns last 15 seconds. Gotta play fast.
Season 42(January 2020)
Fight against an Undertale/Deltarune character and earn rewards!
Random reward at the end of the game depending on the result and the difficulty.
So far, the only boss is Sans. He dodges attacks and doesn't take damage unless they have no monsters on the field. He also places cards only when he gets hit and gets a G. Blaster every turn. He is immune to Fatigue DMG.