By inviting gamers to take part in the survey.
Recently in Twitter – the account of the game Undertale, released three years ago by indie developer Toby Fox, a link appeared to the site deltarune.com, where visitors are invited to download a certain installer with the title SURVEY_PROGRAM ('Program for passing the survey') .
After installing this program, the user first does a little survey, but then gets the opportunity to play the first chapter of a new role-playing game tentatively titled Deltarune – an anagram to Undertale, to which this game appears to be a prequel.
Those who downloaded Deltarune noticed a bug in the uninstaller: along with the game files, all other files located in the same folder as the uninstaller are deleted. Toby Fox himself later admitted this problem and advised not to use the uninstaller at all.
There is no other information about Deltarune, apart from this teaser (or, one might say, demo), at the moment.