Most of the exceptional magic weapons (Holy Avenger, Frost Brand, Nine Lives Stealer) require attunement. Given Justiceseeker's exceptional abilities, I think it falls into that category as well. You could make the argument that a sword of Trithereon just wants to be free, but in this case I think the attunement represents you taking the time to get to know the sword and convince Justiceseeker that you were worthy of wielding the blade.Decide whether the item requires a character to be attuned to use its properties. Use these rules of thumb to help you decide:
[*] If having all the characters in a party pass an item around to gain its lasting benefits would be disruptive, the item should require attunement
[*] If the item grants a bonus that other items also grant, it's a good idea to require attunement so that characters don't try to collect to many of these items.
I do think that the attunement limit for 5e characters should be in place. As I mentioned in "The Goal" thread, I think we should be shooting for thematic conversions; For D'Klar -- for example -- that means I'd focus on his role as a battle cleric with his signature magic items being Dwarkarnytor and the Half-plate of Etherealness.