Judging that in an environment with more Fire-type Megas and fewer Steel types, Toxic's utility has risen, this is a residual-damage build centered on Toxapex, paired with Scovillain, which is newly strong against the increased Steel types. Delphox was adopted as the second Mega to handle what these two struggle with, and the default selection was completed by adding a Choice Scarf Meowscarada — strong against an opposing Swords Dance Garchomp and a male Walking Wake.
The default selection is the Mega (Scovillain/Delphox) + Toxapex + Meowscarada, stacking residual damage from Scovillain's burns, Hippowdon's sandstorm, and Toxapex's Toxic to gradually bring the opponent down. The remaining two slots serve different purposes: Swords Dance Aegislash is brought in to break stall against Venusaur and Doramido, which Toxic doesn't work on, while the Sand Attack-set Hippowdon is used in cycle battles that need Baton Pass/Minimize setup and Stealth Rock. Meowscarada's main reason for inclusion is Trick, which it uses to prevent the team from being tied down by a single wall-type Pokémon.