In the DS adventure game Nine Hours, Nine Persons, Nine Doors, the titular nine persons are trapped together, and each given a watch-like device that assigns them a number from 1 to 9. Using these devices, they must proceed through a labyrinth of numbered doors, matching the digital root of their combined bracelets to the indicated value. The digital root is found by multiplying their values together and taking the final digit--for example, the digital root of 2 and 2 is 4, and the digital root of 0 and 9 is 0. The first person to go through a numbered door, the so-called 9th man, blatantly flouts this rule, and is subsequently killed.
answer (rot13): Gur qvtvgny ebbg vf sbhaq ol nqqvat, abg zhygvcylvat. Gur vaqvpngrq rknzcyrf, ubjrire, ner fgvyy pbeerpg.
Steins;Gate 0 is an unusual sequel, in that it takes place entirely during the final act of its predecessor. Due to a single difference in events, the self-proclaimed mad scientist Rintaro Okabe is unable to complete his original mission, and thus makes World War III inevitable. With Rintaro lapsing into a deep depression, it's up to the other members of the Future Gadget Laboratory to save the day! His childhood friend Mayuri Shiina and the time traveler Suzuha Amane team up to go back to the past and (literally) slap some sense into him. The gambit works, though Mayrui and Suzuha are sent hurtling through time as a result. Thankfully, in the final scene of the game, Okabe is shown rescuing them in a desolate part of Earth's past.
answer (rot13): Hz, npghnyyl, gur svany fprar bs gur ivfhny abiry fubjf Bxnor svyzvat vafgehpgvbaf sbe uvf cnfg frys gb sbyybj, fubegyl orsber yrnivat gb fnir Znlhev naq Fhmhun. Gurve erfphr vf bayl npghnyyl fubja va gur navzr, jurer vg vf, vaqrrq, gur svany fprar.