Back in the days of 1.11 and 1.12, the user Astavie released a version of vanilla skyblock, which included - among other things - advancements, more islands, every tree type, a way to get infinite dirt, a nether fortress, and access to the End. The original map can be found here.
There were some terrain generation errors that prevented the map from updating correctly to 1.13, so I have updated the map with a few notable changes:
Drowneds can drop clay blocks and zombie pigmen can drop netherrack
There are kelp, sea pickles, and turtle eggs in the map
There is an invisible ocean monument at position x=448, z=448
A Heart of the Sea is a rare fishing drop
It snows in the taiga biome
9 new advancements and 10 new challenges
(This is not part of the map)
I will update the map to 1.14 when it comes out, and I will release a video soon on how to update your own 1.12 skyblock to 1.13.
101) Craft a dried kelp block
102) Collect every type of coral
103) Hatch a turtle and collect its scute
104) Make a drowned farm
105) Make a guardian farm
106) Craft every type of glazed terracotta
107) Enchant a trident to its maximum (Impaling V, [Riptide III] or [Loyalty III and Channeling], Unbreaking III, Mending)
108) Brew a slow-falling potion
109) Kill a charged creeper with a conduit
110) Launch a creeper-shaped firework
The nether fortress is at x=-86, z=89, and the ocean monument is at x=448, z=448. Other than that, use the minecraft wiki and each other to obtain new blocks and items - I will not tell you!
TL:DR; you modify the "Monument.dat" file in 1.12 inside the "data" folder to trick the game into thinking that there is an ocean monument where there's not. Only then do you convert the world to 1.13, since "Monument.dat" files aren't relevant in 1.13.
In 1.12, there is a folder in the world folder called "data". The data folder contains a bunch of files labeled ".dat," which is actually a type of folder. It is impossible to open a .dat folder except with a special program, like NBTExplorer. In 1.12, there is a .dat folder for every type of generated structure in the game, such as "Fortress.dat", which contains information about nether fortresses, and "Monument.dat", which contains information about ocean monuments.
The way Monument.dat folders work (as well as the other .dat folders) is that when a player generates a new region (which measures 32 x 32 chunks, or 512 x 512 blocks in the x and z directions, often farther than the player can see), the world generator decides where it ought to place ocean monuments within that region based on the seed, and it creates new files inside the Monument.dat folder - one for each monument in the region.
Each monument file inside Monument.dat serves two purposes. The first is to tell the game how to create the physical monument structure once the player comes within render distance of the structure (once the player can see it). The single monument file doesn't actually store block-by-block information for the monument. Rather, it contains several variables that tell the world generator where it ought to place blocks - the world generator has all the instructions on how to build a monument, but the monument file provides the specifications that distinguish it from other monuments.
This is actually a terrible example since 1.12 ocean monuments contain none of these variables (differences between monuments are either a result of surrounding terrain or pure randomness), but the concept is true for structures like nether fortresses, whose differences generate based on the seed.
So the first use of a monument file is to provide the blueprint parameters that the world generator uses to build the structure - but once the entire structure is loaded, each block is saved manually in a different location, so this particular use is invalid after generation. The second use, which the game references after the structure is generated, is to tell the game where it ought to spawn guardians. The mob spawning program searches through the Monument.dat folder and goes "oh, this particular area is part of an ocean monument, so I might try to spawn guardians here!"
Each monument file has a "position" variable, which tells the game where it is located so that the game can spawn guardians at those locations. So in order to place an ocean monument at any location, we can just change the position variable in any monument file. Then the mob-spawner will think that there is a monument at that location and it will try to spawn guardians. Surprisingly, this process is biome-independent, so you can spawn guardians in the middle of a desert if you like.
You can do this in two ways - one that generates the physical structure and one which does not. I chose not to generate the physical structure (creating an "invisible" monument), but I could have made it so that a real, physical monument would appear if the player got within render distance. In the monument file, there is a true/false flag (or several actually, one for each chunk that contains some of the structure) that tells the game whether the monument has been built. If we set the flag to "false", the game will think that the monument hasn't been built yet, so it will build the monument when the player loads the area. If it is set to "true", the game thinks that the monument has already been built, so it will not try to build the monument, thus the monument will spawn guardians but won't have a physical structure, making it invisible.
This only works for 1.12 and previous since the Structure.dat folders are not used the same way in 1.13. In 1.13, structure information is stored in the same area as all the blocks and entities and is in a format that is much harder to modify. The easiest way to do this in 1.13, in my opinion, is to do it in 1.12 and then update the world to 1.13, and the updater will place the structures in the same places as they were in 1.12.
Hey there, it's been a little while but me & a friend will be streaming this about 7 or 8pm (GMT) I can't promise she will be family friendly like myself but your more than welcome to come watch it on any of these platforms just pick your poison & I hope you enjoy.