Sin's last ability puts all counters that were on Sin onto the target creature, not just its +1/+1 counters
"Dash [cost]" means "You may cast this card by paying [cost] rather than its mana cost," "If this spell's dash cost was paid, return the permanent this spell becomes to its owner's hand at the beginning of the next end step," and "As long as this permanent's dash cost was paid, it has haste." If you choose to pay the dash cost rather than the mana cost, you're still casting the spell
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of The Water Crystal's first ability)
It gets extra flavor points for actually winning with dragons
And unlike most abilities that stipulate that the character cant quest after being readied, Eruption says nothing of the sword, making this a pretty strategically advantageous card
Similarly, if you make an opponent the protector of a Siege you control, mana symbols in that battle's mana cost count toward your devotion