Because Tai’s heart is filled with pride and force rather than true courage, Greymon undergoes a Dark Digivolution into SkullGreymon .
While wandering the desert, the DigiDestined are lured into a Roman-style colosseum by Etemon. Tai, having recently acquired his Crest of Courage, becomes obsessed with forcing Greymon to evolve to the "Ultimate" level to defeat Etemon. Digimon Adventure Episode 16
In the Digimon Adventure franchise, Episode 16 of both the original series and the 2020 reboot represent major turning points. Because Tai’s heart is filled with pride and
SkullGreymon is an mindless, rampaging skeletal monster that destroys everything in sight—including Etemon’s tower and even Tai’s friends. Eventually, he runs out of energy and reverts to Koromon, leaving Tai devastated and regretful for his recklessness. Digimon Adventure Episode 16