WWE hasn't announced this, but both Dave Meltzer and Post Wrestling's John Pollock are reporting that The Undertaker is set to make an appearance during this Thursday's Super ShowDown PPV.
The belief is that this is being used in order to start his rumoured feud with former United States Champion AJ Styles heading into WrestleMania. Honestly, this isn't surprising as the Deadman has become a staple of these shows in Saudi Arabia (as the Prince is a fan of 90s wrestling).
The Undertaker's last match was at July's Extreme Rules PPV when he teamed up with Roman Reigns to battle Shane McMahon and Drew McIntyre.
A couple of weeks ago on RAW, Styles called out a number of WWE Legends, including The Undertaker, so his return will likely be in response to that. We can also probably now expect Styles to win the Gauntlet match for a confrontation with the Phenom after that.