The overworld engine is extremely easy to use, and very flexible. You can also have cutscenes in-battle as well!Attacks aren't super hard to program, as you have the ability to use alarms to time them just the way you like! You can make custom GAME OVER screens, fully customizable dialog with effects like shaking, waves, and you own custom made ones! You can have custom items, custom weapons and armor, and even ACTing! You can make new Soul modes, and new attacks for them and more! You can make custom monsters, or enemies. And it isn't limited to just battles, either! There is a whole Overworld engine included!Everything that you could possibly need to make your dream Undertale Fangame is right here.

The various screenshots to your right were all made in the engine.

However, since I am now the one who does it, I am updating the engine. The Undertale Engine by TML is one that many have used to create fangames like TO!Ink Sans Fight, Gaster Blaster Sans Fight, SpookyTale, Undertale: Knowledge, and plenty of others! TML is no longer maintaining the engine, sadly.

Undertale fight simulator gamejolt