Noob Question

[Itami]

Yamaraja
Skill Prision of the Soul Stack with
Mordred
Skill Outrage

if the answer is not what would be the best soul for a fire team with
Yamaraja Brynhildr Amon Beelzebub(idk xD) Eidolon Ifrit

[Kitty]

definitely mordred .. her skill that applies tons of debuffs will work perfectly with yamarajas .. plus yama has different frame debuff to mordreds, so can be 25% atk/def debuff .. bryn for atk buff, amon for block debuff and...beelzebub is nice at the start of the game for any team as she can apply rampage, but can be replaced in the future for another ssr? fire hime...

[Sfay]

Yamaraja and Mordred are both B frame, they don't stack

I would go Joan of Arc first I believe, overall good skillset and Trial by Jury is especially valuable considering the rest of the team

[bigblackcock]

if it's normal amon, then she isn't needed as long as you have enma (yama) as she also have block debuff.
mordred can do a lot of debuff so it'll get enma's 3rd skill stronger, but as said the attack and def debuff are the same.
again, as sais above arc is good with her cut damage and attack debuff, you can also use ambush as extra skill for another def debuff.