Hydreigon is a very big beneficiary of Tersatalization as a mechanic, Levitate + its typing is very useful with Hazard spam being strong this gen, coupled with the Meta having a severe lack of Fairy types. Ghost-Spam from Gholdengo and Dragapult gives some value to its Dark STAB (defensive typing more iffy due to their secondary STAB), and with Sub/Nasty Plot it can capitalize on an extra turn Tera buys it to get some very strong neutral Wallbreaking in. Tera Steel and Poison are the listed types right now, both resisting Fairy attacks while losing Ground Weakness to Levitate, and then either breaking Tera-Fairy users like Dirge and Garg via "fake STAB" Flash Cannon, or completely blanking Iron Valiant + Great Tusk STABs to exploit choice-locked variants for set-up.What makes Hydreigon viable and used in the metagame ? I remember it being very good in SS OU metagame before DLCs
also not sure about if this the right place to ask this question since I don't really talk here
The speed tier shifts are also interesting, as the game on the whole has some very fast sweeper/cleaners but then a lot of other bulky mons that Hydreigon breaks or can capitalize on slowed down like the surprisingly crowded Base ~84-87 Speed tiers for Tusk, Gholdengo, etc.