Undertale creator Tobyfox has released a free followup game / demo called Deltarune for PC and Mac.
Deltarune is available for download via its official website in both English and Japanese language versions.
(Warning: The following content may contain spoilers for those who would like to play Deltarune without further information.)
The game / demo contains a single chapter that totals about three to four hours of gameplay. Characters from Undertale appear.