How does one got about creating an encounter list of enemies or events that would correspond to your own going campaign. I am creating an underdark campaign and have been using the list of encounter options from the DMG, Monster Manual, Monster of the Multiverse, but they have list of underdark creatures in general, and I obviously don't know what every single one of them is, nor do I need include all of these creatures. I want to create somewhat fleshed out area for the players to interact with. Having certain creatures in locations were it would make sense for them to be. I need a good way to create a table of creatures I can steal out of, and know what these creatures are. How do y'all fellow DM create encounter possibilities for your sessions. I am trying to create some kind of loss base work to pull encounters from without having a set encounters that seem more like a dungeon crawl rather than a world of unknown possibilities. I hope this makes sense to someone, cause all I can come up with a jumbled mess.

When it comes to using DDB for this sort of thing, I usually just search by CR and maybe by Monster Type and see what comes up and if anything fits what I am looking for. This sort of works for me because I've been DMing for so long, I have a feel for what I'm wanting. After selecting stuff, I lay it out something like this as I build out the encounter:

I don't run combat-heavy campaigns, so I'm usually able to spend some time carefully selecting each monster I need for an encounter. The encounter builder is my best friend for searching, comparing, and saving ideas.

Lately, though, I've taken to keeping a list in a Word document of monsters I like and what level and plot hook they're good for. Quick and easy reference.

Thanks for the information 

I create my own random encounter tables for some(most) areas in my campaigns. Example: Recently in one of my campaigns the party was traveling through a wilderness area, forest and rolling hills. So I made a table that had a few things that I thought would make sense in the area. Owlbear, pack of wolves, DireBear,  In this region there is also an abandoned fortress that an evil druid has taken up residence in and started corrupting some of the local animals, so in addition to the Owlbear, pack of wolves, DireBear I added corrupted versions of all three to the table also . So if I rolled a yes for random encounter, then I rolled a second d6 to determine the type of encounter, with a 50/50 chance of a mundane encounter with a normal creature or a corrupted one. All of the corrupted creatures had some clue on them that the party could investigate further if they wanted and possible lead them to the source of the corruption. 

Once a creature has been selected from the second dice role it is no longer an option.   You don't need a huge table  keep it d6-d12. Use the encounter builder to sort monsters pick six or so that you like for the specific area the party is in, that should get you through  a session or two.  then do the same thing again and pic 6 new creatures, after a few session you can recycle creatures from the first two table just mix them up a bit.  In the encounter builder you can sort by size, CR, environment, and type(monster, aberration, ect) this if used well can give you a very specific range to pick your table from. 

Example of my table, rolled on a d6.  

1. Owlbear

2. Pack of Wolves 

3. Direbear 

4. Corrupted Owlbear

5. Corrupted Pack of Wolves 

6. Corrupted Direbear 

It is worth considering as well what each of the encounters adds to the game. The underdark is a dangerous place, so there will be an amount of "a hostile monster appears, roll initiative!", but these encounters can become stale after a while. I threw a random forest encounter into my game, and the players all agreed that it felt a little off compared with the other encounters I had used before.

What I would do is consider monsters which change the mechanics of the game, or employ scenarios where the party has to adapt to achieve what they want to get done. Examples I've used:

• a roper which lives underwater, which dragged a party member into the water.
• an encounter with snakes, in a narrow crawlspace where the party can only engage whilst prone.
• A Cave Fisher dragging a party member over to its cage, to try and eat them through the bars.

The goal is to make the combat in some sense a puzzle - where the answer isn't "target the enemy until the enemy is dead", because the enemy is under murky water, or they are on the roof, disguised as a stalactite, or the party can't see them without compromising. Maybe the air is flammable, or there is a forgotten golem which attacks anything which passes, and which is foreshadowed by seeing an enemy splattered. 

I would recommend looking through the rlevant monsters on the lists you have and think "how would this be cool or challenging to fight?". Further examples off the top of my head with a few monsters on here (filtered for "Underdark"):

• A room with several pools, some of which are gelatinous cubes
• An encounter with a dead, valuable creature (EG a Dragon). The corpse is infested with Carrion Crawlers
• A room with a tile puzzle - don't step on the wrong ones, or you'll get hurt. The ceiling is home to several Piercers, which can drop on the wrong tiles.
• A Fomorian, who's waist-deep in a crag and can't get out. They are hostile. You need to get past them to escape - but they prepare a grapple attack.
• Black Pudding, in a narrow corridor with no manouvering room.

I thank you greatly for that information. Adding non combat items to the list isn't a bad idea. Throughout the adventure they will be uncovering the ruins of this ancient civilization, so perhaps using items to advance the progress though puzzles and mystery. 

As well including interesting terrain, hadnt necessarily though of things like that yet, but I really like it, being in cramped quarters were movement may or may not be available. 

I have been researching the underdark creatures to decide what to play where, under reasonability, but I had gotten over whelmed form the amount of creatures 

I would note that Xanatar's Guide to Everything has random encounter tables by terrain and overall threat level.

The search interface for DDB could be improved -- for example, use a full text parser on monster descriptions to generate a tag cloud so you can find similar creatures.

Yeah I am aware of that table, but it is too random for my purpose, but I have used it before. It is nice to look at and see what monsters would perhaps populate the area 

My primary group is currently dealing with an undead infestation in a forest: it's part of one of the character's back story. 

I built a random encounter table by working off of one I found in a resource of DMs Guild if I remember correctly, but I swapped out a lot of the beasts with undead versions (many homebrewed) and added some third party blights I'd found and created as private homebrew.  I also included potential allies in the list.  This list is created in Dropbox paper, and each entry is linked to the monster entry in DDB (whether official monster or homebrewed).  One could do the same kind of think in One Note and likely other web apps as well.

When we were getting ready for them to enter the forest, I made a batch of rolls to determine what the first several random encounters would be, and set them up as encounters in DDB.

When I was first designing the forest, I found project on kofi: two sets of 30x30 tiles that fit together in a 7x7 square: one set was "basic" forest, the second set added a feature to each tile (e.g. a fey statue, a cave entrance, spider webs). In a few cases, there were additional maps (e.g. for a dungeon accessed through a cave entrance)  You could mix and match the tiles, which I did to build a custom map.  The tiles with special features and a few others I added specific things too (monsters, puzzles, etc.).  If it involved monsters, I built an encounter for it.  For most of the "normal" tiles I didn't make any special notes.   In most cases, entering a tile provokes a roll to see if a random encounter happens; if so, I consult my list of pre-rolled random encounters.