Sub-events are little bits of dialogue that trigger when you're exploring a dungeon, so long as certain conditions have been met. There are 108 Sub-Events in the game. You get an Award after seeing 40, and 100 of them. There is no reward for seeing all 108. It's only really for completion, and provides no in-game benefit. You can skip 8 of them if you want.

Every event needs certain characters to be in your party to trigger. Although most of them are just for a bit of fun, some reward items or buffs. There are also a few that actually inflict negative status effects on your characters. Even if all the conditions are met, you usually have to walk around a bit before an event triggers.

This guide is ordered by character in the order that those characters are obtained. If a character is needed for a sub-event, and that character is obtained before the other characters required, the event will be listed in their section. I've also included images of the completed gallery pages at the very end. If you are looking to fill in specific empty slots on your page, please reference these images and CTRL+F the names of the events.

If an event says 'After Battle', it just means you have to get into battle and fleeing counts. This means you can simply stand in front of a guaranteed encounter door and spam forward and escape to quickly rack up the required battles. If it says 'After Victory', it means you actually have to see the win screen.

Some Sub-Events award stat buffs to certain characters. They are as follows:
Four-Leaf Mandra - Tamaki Skill Points + 10
Silfa's Maintenance - Silfa AGI + 20
Lone and Level Sands - Karin RES + 20
Harumi Redux - Harumi HP + 20
Voices Within - Yuma TP + 40
Luuko and the Mini-Uu - Nanako INT + 20

All of these buffs are permanent, except for Tamaki's. I reiterate, she will lose the additional Skill Points if you reset her level. Not that you really need more Skill Points at this point.

Most of the information is taken from the JP wiki https://w.atwiki.jp/th2dt/pages/124.html alongside my personal experience with triggering these events. I'd go as far as to say this is basically just a translation of the Japanese wiki. All credit should go to the wonderful contributors on the JP side.

* Four-Leaf Mandra are dropped by Meta-Mandra♂ (Tower 27-28 and 30, Dark Cathedral)
** Genjimaru is summoned with Konomi's unique skill. Have Konomi use a Fish Sausage in battle. Summoning him once isn't guaranteed to trigger the event, I had to do it multiple times (but I was going for the Award for summoning him 10 times anyway).

* I would recommend doing "Get Rich Quick!" early. It is very tedious to burn through your money in the post-game. I started a new save file just to get it in the gallery.
** Slime Jelly is dropped by Slapyon on Floor 1.
*** You can trigger sleep by using any Daki item outside of battle.

* You can inflict poison on yourself with Poisoned Meat dropped by orcs from the Battle District. Or just step on a poison trap
** You can inflict slow on yourself by using Slime Jelly

* Silfa Daki is dropped by Golem on Floor 5

* Striped Panties are dropped by Sniper on Floors 18-20.

* Megiddo's Tome is dropped by Forbidden Text in the Dark Cathedral, and by Cintlalinicue of the Four Heavenly Kings (guaranteed encounter on Floor 20). It's a bit hard to come by, so be careful about accidentally triggering this event if you want to keep the tome.

* This is how I got it to work. The JP guide just says have her die twice in the tower, but that didn't seem to do anything for me.

* Power Armor is dropped by Leopard on Floors 9-10

* Lustrous Mist is dropped by Mist Dragon found in the Thirty-tier Tower from floors 16-30.

* Kraken can be found on Floors 11-12 and 14-15.
** Octoplush can be found on Floors 17 to 19 and Royal Tomb 1, 2, and 4.
*** Raatsikamuy can be found in the Thirty-tier Tower floors 6-15, 21-30

* Yashiori Sake can be obtained from Orochi, Floor 16, Royal Tomb 1-2.

* Kazusa Tee is obtained from Morrighan in Dark Cathedral.

* Dark Power can be obtained from Saturday Knight on Floor 16, and the Royal Tomb.

All entries from the Magical Monster-y Tour will automatically trigger while you're exploring. You should naturally have all of them by the time you complete the main dungeon.