So I was talking to this guy and he and I agreed upon the fact that this game needs events to keep players entertained, or else it gets boring rather quickly. So here is my suggestion for the Halloween one, although its like 3 months away.
(P.S. I've been told that Halloween isn't really popular outside of America, but still! *is American*)

So I've been thinking...what if Shilly's house was raided by ghosts on the month of October?
Maybe the event exclamation mark could be red instead of yellow or green.

Anyways here is how we start out. First we come to Shilly, and (although this part is kinda obvious, we click on the [Event] quest somewhere in his NPC available quest list) he tells us that he is trapped outside of his fortress because, well, ghosts came in the middle of the night and chased him out. So he asks us to go in there and cleanse the place of monsters (because we're so darn good at it. ), and also to investigate where the heck the monsters came from. Terminus is always night, so the players should be able to do accept it at whatever time of the day as long its once a day. So kinda like dungeon dailies, as you can accept them once per day but you can still run the dungeon without them.

And so, after we accept the quest we enter his living place (dungeon) through a portal stationed at his front door, which looks kinda like this:

(I think it will be neater for Shilly to be an entrance NPC for his own home dungeon, so that when we are in a party we can do it with friends on different modes)

After we enter, we see ourselves in his living room (he has the usual, the couch, a fireplace, bookshelves, the stairs to upstairs, a clock, etc) which I imagine it cozy and dark.

Like when a GM tells the whole server hi, orange text appears on the player's screen saying, "Huhu...welcome. You've fallen right into my trap..." and after a second later this 'voice' says more, "I've hidden a special box... if you can find it, I'll let you out."

Aaaaand with that the player starts searching and examining the room. Since this is Halloween, I'm thinking maybe that big self portrait in the room has holes in the eyes, as if someone was trying to watch their every move. Or maybe the smoke coming from the fireplace are weak ghosts that just dissipate in to the air after they come out. There can also be books with legs walking around the room telling the player to watch where they're going, too.

We find a glowing book poking out on the bookshelf under the stairs, and after we "loot" it, the fireplace is flipped sideways like a hidden door. Enter that, and we find ourselves in a strange corridor that definitely was not an original part of the fortress.

Since I noticed this game puts a lot of details into one thing, maybe we can add something like black transparent hands on the corridor's edges so it looks like they are looking for the player to grab, or ghosts floating around mumbling something about the lack of spiders to make cobwebs to add a spice to the area. Either way monsters are summoned whenever we walk a certain distance (for solo mode it can be that it only summons 1-2 monsters, while in harder modes for parties there can be as much as 4-5). The monsters might as well be something ghastly, such as:

-Wandering Souls (Shrouded Vallley)
-Zombies (also Shrouded)
-Ghouls (Hidden Cavern 2F)
-Barren Treeky (Wind Basin, correct me if this is wrong)
-Wooly Spiders (Corrupted Valley)
-and anything else you guys can think of.

Reaching the end of the corridor the player finds another door, and we enter that one too. We find ourselves alone in a small room with three chest-like boxes laid out in the centre (or five if you want to lessen the probability), each emitting a different coloured aura. And by aura I mean the colours you see on the loot box when you get common, epic, or unique items. We take a guess and loot one.

If we choose the right box, something like a tarot stall appears on our screen. The cards are flipped for a second and as we always see in normal tarots; they are flipped back and mixed after a glimpse. But these aren't normal tarot cards, oh no. By clicking the cards our reward from clearing the dungeon (whether it may be a piece of costume, food, equip cards..) are decided. After that you are dragged out of the dungeon, as soon as it displays what items you got from choosing, like how you are teleported out of Vortex after seeing what you got.

However if we happen to choose the wrong box, we are teleported to another room depending on what type of a "wrong box" we might loot.
For one box it might be that Habetu (the witch that is mentioned in Nightfall Shelter and Corrupted Valley) becomes the boss you must fight.
For another box it might be that you are teleported to a labyrinth, where you can NOT click M and fight your way through. Maybe the labyrinth has some sticky debuffs the player might get if he runs into a dead end or ends up meeting a monster inside.
If there are five boxes, it can be that the player is moved to a place where he has to dodge upcoming arrows or bullets from monsters from the other end of the room. These won't do much HP damage but rather push you back a little, as you have to walk up to the monsters and kill them---with the other box having a "HAHA, NICE TRY BUT GO BACK TO THE BEGINNING OF THE CORRIDOR AND WORK YOUR WAY UP AGAIN" thing you have to deal with (meaning you are teleported to the first steps of the corridor and have to fight your way through monsters again).
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After clearing their outcome from choosing the wrong box, the tarot-like thing appears on the players' screen and after choosing the card for rewards, they are teleported out.

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For the costumes you can get, I was thinking maybe creating new ones that have witch(if girl) or wizard (if boy) look that reminds you of Halloween, but meh. The costumes might not even be permanent, as I commonly see this on other games' event costumes.
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Thank you for reading! /o/