Basic Gravestone

First, you'll need to create the base of the datapack. The structure of the datapack should look like this :

└─ data
  └─ [namespace]
         └─ gravestones
               └─ gravestone1.json
               └─ gravestone2.json

The Structure

Here what a basic gravestone looks like :

{
  "id": "graveyards:arachnophobe",
  "level": 1,
  "monsters": [
    {
      "type": "minecraft:spider"
    },
    {
      "type": "minecraft:spider"
    },
    {
      "type": "minecraft:spider"
    },
    {
      "type": "minecraft:spider"
    }
  ],
  "rewards": {
    "experience": 3,
    "loot_table": "minecraft:chests/ancient_city"
  }
}

id : the resourcelocation of the gravestone

level : at which level of gravestone this gravestone can be started

monsters : a list of monsters that will spawn

Reward

loot_table : the loot table that will spawn when the player will finished the gravestone

experience : the experience level that the player will receive