(Sorry for bad english, :p)
(Please don't say that MV or VX have already an Undertale system, I know, thanks.)
Engine: RPG Maker XP
A fan-sequel/game of Deltarune, a game by Toby Fox.
DELTATALE is a game with an engine made from zero using RPG Maker XP and Ruby systems.
[I feel a bit sad I did this]
"WhY?...WhY dId YoU KiLl ThEm AlL?...yOu BlOoDy HeLl MoNsTeR...I'LL MAKE YOU PAY!!! FOR THE BLOOD OF THE LOST!!!" ~Scars final words before his final battle [not final boss]