What is it ?

A mod which features a bunch of useful gadgets for mapmaking and modpack creation/customization which allow configuration of various things through comprehensive items and GUIs which do not require you to leave the game at any point. Mostly useful in creative mode

What does it offer ?

Mapmaking and modpack customization tools , which can be used to tweak entity equipments/mounts/persistence, force mobs to infight, make custom potion combinations and configure mob spawners;

Here's a list of possible things you can do using this mod :

-Make a custom villager using the villager trade editor and spawn him again somewhere else
-Add a description and tweak vanilla attributes on an item
-Make an invulnerable, silent and invisible creeper which can see you 75 blocks away and has the same drops as a zombie
-Make a splash potion with the color of your choice of both jump boost and instant damage with a somewhat infinite duration and no particles then throw it on a mob
-Save a customized zombie who is named Jerry and is wearing a dirt block on its head into a spawn egg to make endless copies of him
-Configure a spawner to make it activated when a player is 5 blocks away from it and can spawn periodically hordes of zombie pigmans and cows in a 100 block radius
-Save an entity into a tiny spawner which will conserve most of its data, turning the block back into a mob when a survival/adventure player is near
-Copy the NBT of an item or mob to save it for later use
-Make two stacks of stray and have each one target one from the other stack (has to be done one by one)

I like the way this mod is presented to the player and find the GUI very intuitive (I have not found any issues with the performances);
Only nitpick I have is that every time you use the entity editor on a mob and get to the tab with a wrench it seems to automatically reset the vanilla loot table, which means I have to add it again; thankfully I just have to do it once then save the mob in a custom egg and it's done.

In conclusion

This simple mod is a very useful one , I have been using it in creating recurrent complex custom structures as it allows me to easily prevent mob from despawning or place mounted entities in tiny spawners to make sure recurrent complex saves them right.

While probably made with 1.12 map making in mind, it is a very powerful tool in modpack making , that I will keep recommending.

If you are looking for alternatives/more recent versions of the mod I have yet to find one, but Mob Battle allows you to tweak mob equipment/make custom mounts/save custom eggs, while being better at making custom mob infighting at the same time (but doesn't feature deeper entity tweaks, you'll have to look for mods like Dynamic In-Game NBT Editor , among others that I don't know of which are probably even better).

Remember to check the Modpack Customization Tool feature tag for more mods like these, and to suggest it for any mod which might make modpack creation easier.