java.lang.IllegalArgumentException: Erebus can not use biome ID 100 because it's being used byam2.worldgen.BiomeWitchwoodForest@316461c2. Please choose a different one.
Two mods are trying to use the same biome ID. You need to go into the config file for one of them and change it so it doesn't match the other. Try to find an ID that isn't used by any other biome. You can start with 101.