Sephiroth's Dark Heavens
Sephiroth's new weapon is called the Dark Heavens. It sounds like a Kingdom Hearts weapon, and honestly, Sephiroth here reminds me of Riku. Dark Heavens has the Gale Strike skill, which deals single-target physical wind damage at a 4 ATB cost.
At level 90, it has a modifier of 440%, at OB6 660%, and at max, 800%. It has good modifiers and is on par with some of the single-target damage weapons in the game. Nothing game-breaking, but good nonetheless.
Its R abilities are quite good, offering physical attack and wind potency bonuses at 24 points and 9 points at level 90. At OB6, it gives 34 and 24 points, respectively, and at max Overboost, 40 and 36 points. I'm really happy that it provides physical attack instead of just "attack." I can't get enough physical attack weapons for Cloud and Lucia's physical damage builds.
For support materia slots, it has 2 physical ability damage boosts of 20%, and the last slot provides an O Sigil boost. It is similar to Cloud's Maritime Sword in terms of support materia slots.
Its stats are geared towards physical attack, thankfully not like some weapons that don't match the type of damage they do. And by that, I mean Lucia's Serpent Eater.
Notice that this deals physical damage.
Since Sephiroth has always been built as a magic damage dealer, this will require a bit of investment in physical damage secondary and sub-weapons. If you have Cloud as a physical damage dealer, you can equip Sephiroth with these weapons as sub-weapons to boost his physical attack. You can equip Seph with Glareid as his sub-weapon to take advantage of the physical attack boost.
Knowing this, it's a good weapon to draw for and invest in. It's not that awkward to turn Sephiroth into a physical damage dealer, and the game does need a wind physical damage dealer with a corresponding outfit to boost wind damage. The only other wind physical single-target damage dealer is Tifa with her Motor Drive. Sadly, she doesn't have an outfit that boosts wind damage.
Speaking of Motor Drive, it would make a great sub-weapon for a wind Sephiroth and vice versa.
Sephiroth's Dark Harbinger
On to Seph's outfit, the Dark Harbinger. I love this one as well. I guessed correctly when I first saw the teaser trailer that it would be Seph dressed in a Behemoth costume. It is well-made, and I like it better than Cloud's outfit.
This one boosts HP and wind mastery by 10 points. The wind mastery only provides a 20% wind damage boost, unlike Cloud's outfit that boost lightning damage by 35%. Still, I'll take the 20% over nothing at all. This outfit has great synergy with his weapon, the Dark Harbinger. It is best in slot on a wind physical build for him. It could have been better if it had a physical attack boost instead of an HP boost. However, that is just a small nitpick.
I would choose this outfit as the 12-stamp reward.