It can be unlocked at lvl 40, and Samurai, Ninja and Conjurer should need soul to unlock it, like the previous heroes. Also add a t5 dice for it (the fusion for made these dices could require the new supreme moonstone)
#5º slot for heroes skills
blissful palm
raven seal
I like this idea. You could even include a new set of skills exclusively for this slot like "Legendary Skills" that require more elemental affinity.