While the Legendary rule is not explained on the cards, it is explained on the Tips'N'Tricks cards inserted into packs
I often snap these off as first picks, as they are far more important to the aggro archetype than any Isamaru, Hound of Konda
It must have been challenging to translate/adapt so many of these huge concepts that take up literal pages, and entire books, into (mere) Magic cards
Xenk's sword is inscribed in Celestial with his oath, Neither Virtue Nor Blade Shall Break. it's a tangible reminder of who he is and what he believes
Esper Valigarmanda is a solid Saga enchantment, but combining it with Esper Terra can lead to significant advantages in red aggro decks
When it goes off, it potentially creates an increasing feedback loop that just creates more spells, more cards, and more mana