I took a break from Heroes for a while, but I recently got back into it and feel like sharing my +10 units:
Despite being a 5* exclusive unit, Sigurd was actually my first +10. I got quite a few copies of him from the Hero Fest banner right when I first started playing, picked up a few more as pity-breakers, and then started actively pulling on banners he appeared on for those last few merges. Steady Stance 4 bolsters his already good Def and grants him Guard when initiated on without taking away his B slot. Divine Tyrfing, Spd Smoke, Darting Stance, and his Spd asset allow him to tank hits from most mages, with Divine Tyrfing cutting the first hit in half and his Spd advantage preventing regular follow-ups. Crusader's Ward doesn't get too much use with the preventing doubles plan, and I hope to replace it with Lull Atk/Spd (better defense) or Lull Spd/Def (better offense) if I manage to pull either in the future. Dragons and staff units are his obvious weakness, but he fares well against everything else. He's on my main cavalry team with Reinhardt (Ally Support), Legendary Gunnthrá, and Brave Veronica. He'll have some Dragonflowers headed his way as soon as I have enough.
My second and third +10 units, Marth and Soren, were for my Arena core:
I got Marth to +10 right after Sigurd. His entire build is focused on support. His unique Falchion refine along with access to the full suite of Drive skills in his C and S slots allow him to support any allies within range, with his B slot providing Chill Def or Chill Res depending on his allies' needs. Summer Helbindi was a blessing for him, bringing along Rally Atk/Def+ and R Duel Infantry.
Soren's build is similarly focused on support. He provides the two missing Chill skills with Chill Atk on his weapon and Chill Spd in his B slot. He also provides additional support with Rally Up Atk+, Joint Hone Spd, and another Drive in his S slot. Blue Flame fits well with the special refine on his Wind's Brand. Both Marth and Soren have Spd assets.
The third member of my Arena core is Legendary Azura, and I hope to get some merges on her next time she appears.
With my recent return to Heroes, I finished my fourth and latest +10, Ares:
Ares is an absolute monster. Dark Mystletainn is absurdly powerful. Slaying Special Spiral results in a permanently active 3 CD Special after first combat, with Bonfire being the Special of choice. Distant Counter Vantage ensures that Ares always hits his foes first, regardless of phase or range. Def Smoke softens up any nearby targets and Brazen Atk/Def stacks even more points onto his best two stats when in Brazen Vantage range. With Brazen Atk/Def active and Atk Tactic and Def Tactic support from his allies, his Atk reaches a ludicrous 72 and his Def a very sturdy 50. Add another 2 Atk from Legendary Hector's Fire Blessing and take into account the potential -7 Def penalty on foe, and he'll hit for 74 (Atk) + 25 (Bonfire) + 7 (Def Smoke) = 106 total damage. Few units can survive that.
Ares is on my main team with Legendary Hector (Fire Blessing and Ostia's Pulse support), Ophelia (Def Tactic support), and Adrift Azura (Wings of Mercy Sing and Atk Tactic support).
My next 5* exclusive +10 merge priority is Helbindi:
Helbindi helped me out a ton in the Abyssal Legendary Alm battle, tanking the entire right side on his own without much issue. Distant Counter and Null C-Disrupt allow him to counter at any range and through Firesweep and Dazzling weapons, which works well with Quick Riposte for fast Aether charges. Hack-o'-Lantern+ provides Guard and its +Res refinement pulls his Def and Res stats satisfyingly even. Atk Smoke neuters nearby foes' Atk. His Spd is abysmally low, but with Guard on his weapon and his excellent mixed defensive stats he can take the extra hits, and perhaps even appreciates their help in charging his Aether faster. With 5 more merges, 5 more Dragonflowers, and Summoner Support, he'll hit 60 HP and 43 Def and Res.
I'm also looking for any merges I can get on my Legendary Hector, Legendary Azura, and Ophelia.
My next two non-5*-exclusive merge projects are Cherche and Est. I already have +Atk copies of both promoted to 5*, but I'm a long way off from having enough Feathers to +10 merge either of them. I'm also still looking for a +Atk Reinhardt for his final merge. I still haven't decided which Grail unit I should prioritize for merges. Black Knight fills a unique speedy red physical armor role which I currently lack, but I don't use my armor team much at the moment and he alone would not change that. I have a Winter Cecilia with Close Counter who would benefit a lot from merges, but the same goes for her. Aversa is a permanent member on my Aether Raids teams and she would benefit a lot from the extra HP, Atk, and Spd, but she would remain mostly restricted to that mode and I don't urgently need the extra stats on her. Adrift Azura is by far the Grail unit I use most in PvE content, but I am unsure if I should invest so many resources in her.
Outside of merges, I tried to pull for Tibarn for Sturdy Impact and managed to get two (after a pity-breaker Zelgius): one +Def/-Atk (worst flaw), one +Res/-HP (worst asset). I think I should keep the first one for now and use the second for skill inheritance. I plan on inheriting Sturdy Impact to Est as she will be initiating on melee armor units, for which both the extra defense (reaching 40 at +10 with Attack/Def in her S slot) and the follow-up denial (shutting down Vengeful Fighter and Quick Riposte) seem useful.
Death Knight GHB was trivial. Looking forward to the next Legendary banner for some more Abyssal content.