This is Undertale Customs, a fangame series where you face off against monsters, and have the choice to either spare them or kill them.
Current Version (1.3.0)
Please check the included readme file in the game folder. If the current version of your game is outdated, download the current version on this website or on gamejolt, and replace the files previously downloaded with the ones you have just downloaded, in order to get the latest content for Undertale Customs.
Please check out my Gamejolt page for news on Undertale Customs: