How make a world, that will be all spreaded by evil surroundings? Not covered from start, but fully dominated by goblins and necromancers.
You may want the raws for that one, specifically entity_default. There's only a few things you'll be interested in; I'd suggest making a note of them (just type something out next to them, like their original value, to make them noticeable) so that you can change them back to normal later without copy/pasting entity_default over itself. Keep your changes organized and well noted, or your raws will become a mess and you'll have to either redownload and steal some fresh raws or copy/paste unchanged raws from the wiki, both of which are annoying.
[MAX_STARTING_CIV_NUMBER:3]
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
You'll want to look at this block on every civ. You can just turn off the races you don't want with MAX_STARTING_CIV_NUMBER, and by setting MAX_POP_NUMBER to 1, you can get dwarves to instantly die out but still normally be playable. I don't typically tweak MAX_SITE_POP_NUMBER, but it's still in the trio, I guess. At the very least, set MAX_STARTING_CIV_NUMBER to 1 on every race except goblins, you'll need as many goblin civs as you can get.
Next up, you'll need to make goblins able to make necro towers. They need to both have gods (to bestow the power) and be mortal, which requires a brief trip to creature_standard. Just CTRL-C/CTRL-V the goblin religion spoiler into their entity, and give goblins a [MAXAGE:999999:999999] in creature_standard, which keeps them effectively immortal but still technically mortal for our purposes.
[RELIGION:PANTHEON]
[RELIGION_SPHERE:CHAOS]
[RELIGION_SPHERE:DEATH]
[RELIGION_SPHERE:METALS]
[RELIGION_SPHERE:DEFORMITY]
[RELIGION_SPHERE:DEPRAVITY]
[RELIGION_SPHERE:DISEASE]
[RELIGION_SPHERE:WAR]
[RELIGION_SPHERE:CAVERNS]
[RELIGION_SPHERE:DARKNESS]
[RELIGION_SPHERE:MURDER]
[RELIGION_SPHERE:TORTURE]
The last thing you need to do is the most complicated. Look at the block of settings that contain the information regarding biome support.
[EXCLUSIVE_START_BIOME:MOUNTAIN]
[SETTLEMENT_BIOME:ANY_FOREST]
[SETTLEMENT_BIOME:MOUNTAIN]
[SETTLEMENT_BIOME:ANY_GRASSLAND]
[SETTLEMENT_BIOME:ANY_SAVANNA]
[SETTLEMENT_BIOME:ANY_SHRUBLAND]
[BIOME_SUPPORT:ANY_FOREST:1]
[BIOME_SUPPORT:MOUNTAIN:3]
[BIOME_SUPPORT:ANY_GRASSLAND:1]
[BIOME_SUPPORT:ANY_SAVANNA:1]
[BIOME_SUPPORT:ANY_SHRUBLAND:1]
[BIOME_SUPPORT:ANY_RIVER:1]
[BIOME_SUPPORT:ALL_MAIN:1]
[BIOME_SUPPORT:ANY_RIVER:1]
For dwarves, it looks like that, but just keep an eye out for the BIOME_SUPPORT tags, they're the ones you want. Remove some of them; I like to just put a -- in place of the first open bracket of each thing I want to remove, which nullifies the tag without permanently deleting it, so I can undo the changes later. This requires some tweaking, re-launching the game, and seeing how civs spread. Your goal is to limit the spread of elves, dwarves, and humans (without killing them, unless that's what you want, in which case don't fiddle with this, just set starting civ number to 0 or max pop number to 1) so that goblins can truly dominate. It's not entirely necessary for your goal, but it will help make it more likely. I normally set dwarves to only settle in mountains if I want to limit their spread, and humans and elves can just have a few random biome types poked out of their biome support to slow them down.
And then you're basically done! Crank up the civ count as high as you can without placement errors, turn down max site cap and max pop cap (so you can gen your world for a long time without freezing every year), and watch the evil spread! The basic idea should function in almost any type of world, so you don't have to worry too much about your parameters affecting your goal in a detrimental way.