This team is a relatively standard build with Scarf Blacephalon and Mega Latios, a core that
Gary outlines
here. It is made predominantly for post-Naganadel metagame, which is why it differs in structure from the team posted above by Bribery (his has Scarf Greninja and Specs Blacephalon whereas mine is the opposite), that is otherwise quite similar. You can still manage Naganadel temporarily, but it is not going to work well if the opponent executes well in all likelihood. Regardless of that, I think that this team is solid and will function quite well, especially after the likely impending ban. Just to go through the sets and synergy:
- Blacephalon is the standard Choice Scarf set, but it has Explosion over what could be Will-O-Wisp, Knock Off, or a second Fire move. This lets you assure an OHKO on Volcarona, which you will likely need to revenge kill when facing. In addition, it can function as a nice emergency exit button later on in games when Blacephalon is no longer needed, but it is hard to safely pivot in to Kartana or pre-Ash Greninja while leaving the opponents Pokemon alive.
- Mega-Latios is there in order to serve as a Tyranitar lure, but also do all of the things Latios generally does. It is a nice special attacker with a practical defensive presence that hits hard and fast. Earthquake on this thing compliments the Blacephalon very well and the additional bulk the mega evolution grants it also makes up for how fragile Blacephalon is sometimes, too.
- Landorus-Therian is filling the generic Stealth Rock role on this team, but there is a bit more to this set. I have been using this variant of Landorus-T on some teams ever since the middle of Snake and I like it a lot because of the Knock support that it gives in the Celesteela match-up, making it infinitely easier to kill or chip to support Mega Latios and Kartana. It also takes pressure off of Blacephalon as it currently has to lock in to Fire Blast to make progress/force out Celesteela currently, which can potentially lead to getting outpredicted and trapped, giving Heatran a Flash Fire boost, or even letting Toxapex come in to wreck havoc in an ungodly fashion. On top of this, I have added Speed to this Landorus-T in order to outrun Adamant Zygarde, which the team otherwise is vulnerable to defensively, and Heatran, which is especially nice considering it gives another form of support for Blacephalon in case Mega Latios is not enough. I feel U-turn is not necessary here as the team lacks Pursuit and can function on hard-switches. The SAtk is to 2HKO opposing Leftovers Landorus-T with Hidden Power Ice and the SDef is for surviving Mega Manectric HP Ice from full. Before you ask: Yes, all of my Landorus-T spreads are this complex.
- Next member is standard AV Magearna, which is nice to check things like Tapu Lele, Latios, and even Greninja in the early game (although I do feel this team might be a tad more Greninja vulnerable than I would like it to be and that might be because of the core of Blacephalon + Mega Latios being Greninja food). I am running less speed IVs than normal not only to assure that I underspeed opposing Magearna, but also to outslow Chansey, giving me a clean Volt Switch to whatever I choose. I have ran this spread for a while and I find it to be quite effective in the fat match-up. Additionally, I go with Iron Head to help me with opposing CM + SG Magearna, that otherwise can make quick work of my team.
- Specs Ash Greninja is a very standard set and a pretty easy addition to the team, giving me a soft, temporary check to opposing Blacephalon, some priority to help on the revenge killing front, and even Spikes, which can be very helpful in the balance or fat match-ups.
- Finally, SD Kartana with Defog is the last member and sort of a glue Pokemon for this team. It helps a lot against bulkier teams, but it also provides arguably necessary Defog support and a hard hitter to nuke things like Clefable, Landorus-T, etc. I wanted to use Grassium as a 'kill button' in a sense, but also because I knew that my team played in to Knocking Celesteela with Landorus-T, so I was aiming to chip it into range for +2 Sacred earlier on regardless, making one of the biggest Z-Sacred Sword targets taken care of in the team's gameplan (assuming it is executed properly). You could likely use Z-Sacred Sword if you wanted to in order to have a safety net in this sort of match-up, but generally I prefer the nuke on this sort of team with a STAB Z-Leaf Blade, so I went in this direction. A potential OO would be making this Z-Sacred Sword with Knock Off > Swords Dance and then making Landorus-T Protect with Leftovers over Knock Off with Rocky Helmet to give more longevity against things like Zygarde, but I like this variant much better against bulkier teams, so keep that in mind.