At long last. After a solid eleven months of the only Radiant Dawn representation in the game being Sanaki, Intelligent Systems finally takes pity on all of us and gives us Micaiah. Rejoice, for our judgement day has come.
Because Micaiah is busted.
Micaiah: Priestess of Dawn
The first unit in the game to have effectiveness against two unit types, armored and cavalry, Micaiah shines right off the bat with her legendary tome, Thani. Additionally, against ranged armored and cavalry units, the damage from the first attack is reduced by 30%, giving her more bulk against units like Brave Lyn and Reinhardt. Thani alone gives Micaiah a must-have spot in Arena teams, especially against cavalry or armored teams, but her power is further improved on by her high Atk stat of 35 and high Res stat of 35, making her the ultimate armored and cavalry mage counter for all those pesky Christmas Tharjas that Reinhardt can't kill.
Micaiah comes with the games first unique assist skill, Sacrifice. Sacrifice functions as a combination of a more powerful Ardent Sacrifice and Harsh Command, in that Micaiah can heal her allies by her current HP and convert debuffs into buffs. With a Renewal B-slot, Micaiah can become a powerful healer, which can be useful for chain events like Chain Challenge, Squad Assault, and Tempest Trials. Distant Defense is also a good skill for Micaiah to have, as against mages, she will sport a whopping 44 Res, making her virtually unkillable by magic. Her default B-slot, Guard, also helps to prevent her from being nuked by a surprise special, though its high HP threshold is a little hard to maintain.
Micaiah's main weaknesses are in her Spd and Def. True to her original incarnation in Radiant Dawn, Micaiah has a relatively low Spd stat of 28 (that can be patched up somewhat with Quick Riposte), and is extremely physically fragile at 18 Def, meaning that she has to be wary when attacking units with Distant Counter or weapons with DC built in like Dorcas or Hector.
Why should you summon?
You've had enough of Armor and Horse Emblem. Micaiah is an extremely good mage counter because of her high Atk and Res. Her chibi sprite has Yune on her shoulder and you must have that in your life, immediately.
Why shouldn't you summon?
Micaiah is slow and fragile. You already have a handful of Armor/Horse Emblem counters and don't really need another one. Glass-cannon blue mages work better for you.
Sothe: Zephyr
Actually good dagger units are a rarity these days, and Sothe is one of them. In many ways, Sothe is a glass cannon. While his 36/33 offensive spread is amazing and grants him the highest Atk of any dagger unit in the game, Sothe's durability is extremely limited at a meager 22 Def and 21 Res. Sothe is also the second dagger unit to come with a legendary weapon after Jaffar, and his weapon, Peshkatz, has a whole 3 Mt over Jaffar's Deathly Dagger, essentially making him an offensively superior Jaffar.
Peshkatz functions as a better Rogue Dagger in that after combat, an AoE debuff occurs on all four stats and buffs nearby allies at the same degree, allowing Sothe to incapacitate the enemy team while strengthening his own. His C-slot skill, Spur Atk/Spd, buffs his allies further by +3 Atk/Spd, which comes in handy if you need offensive power in a pinch. Finally, his A-slot, Life and Death, solidifies Sothe's role as a glass cannon by raising his Atk to a whopping 41 and his Spd to an impressive 38 but leaves him extremely fragile at 17 Def and 16 Res.
Why would you summon?
Jaffar just isn't doing it for you; even with Deathly Dagger's refinement options, he's still pretty subpar in terms of offensive power. You want a unit that can steal the lifeforce of the other team to buff your own. Sothe is offensively really good, and there aren't many people running counters for dagger units.
Why wouldn't you summon?
Sothe will die if the wind blows the wrong way. You've already invested in Jaffar or New Year Takumi and don't need another dagger. You want to hold off until he's inevitably demoted so you can summon him for Life and Death fodder.
Zelgius: Walking Spoiler Jet-Black General
Keeping in line with FEH's lovely trend of spoiling old games before people play them (*coughs* Seliph and Lucina) is the unmasked Black Knight, Zelgius. Somehow, getting his armor scratched and taking his helmet off makes Zelgius more powerful, because he is superior to Black Knight in Atk, Def, and Res, only falling short in Spd and HP by one or two points. With a good offensive spread of 36/33 and fantastic Def of 38, there is not much Zelgius can't tank and kill in the way of physical weapons, since his sword, Alondite, allows him to counter at any distance. However, though his Res stat is slightly higher than Black Knight's, Zelgius' 21 Res won't do him any favors against mages and clerics.
Zelgius has not one, not two, but three unique skills: his sword, Alondite, his special, Black Luna, which cleaves through the enemy's Def like nothing else, and his unique B-skill, Warp Powder. Warp Powder allows him to warp next to any ally within 2 spaces as long as he has more than 80% of his HP, which grants him the mobility that he otherwise lacks as an armored unit and allows him to surprise the enemy. Zelgius is also the second unit to have Fierce Stance, contrasting with Black Knight's Steady Stance and making Zelgius even more powerful on the enemy phase. Finally, his C-slot, Panic Ploy, allows Zelgius to debuff the enemy team as long as he has more than 1 HP than his enemies, which synergizes well with his high 46 HP. Overall, Zelgius is a more powerful and tanky but slightly slower version of Black Knight that you can optimize with IVs and merge to +10.
Why would you summon?
Zelgius' added mobility with Warp Powder makes him an asset to Armor Emblem teams. He's basically a better Black Knight. You're getting desperate for Fierce Stance and you don't have a spare Dorcas. His damaged artwork shows some of his abs and you're all about it.
Why wouldn't you summon?
You're perfectly satisfied with the Black Knight you got from Tempest Trials and don't need another sword-wielding armored unit. If you try to summon for Zelgius, you might get a bad IV rather than a good one, and you're not willing to take that risk. Zelgius still can't take a magic hit without losing significant HP.
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If you have the orbs to spare, summoning on this banner is pretty worth it. None of the units in this banner are offensively lacking and can fill specific roles in your team, whether it be that of a nuke or that of support, sometimes even both. Overall, unless you've already invested heavily in similar units, it's worth throwing a few orbs.