Whenever he or another legendary creature enters the battlefield under your control, the ring tempts you, which carries with it a slew of other abilities
DCC - Moonlight Blade 80+ If this Pokmon has any damage counters on it, this attack does 80 more damage

Finally, as to why Yuma can draw monsters not on his deck, again, it is the same principle (being, again, possibly a superpower explicit in the yu-gi-oh universe, where card games are, if not religion, explicily supernatural, and thus not viable in our world) albeit breaking the laws of physics and being more powerful than normal due to it using more power than humanly possible, (if we assume the maximum a very skilled duelist can produce is 100%,two overlaid duelist can produce 200%, enough to manipulate Schrodinger's law and general possibilities so much that 0% becomes 100%, moreso if it is assumed they use overlay units to activate this effect, and less so if we assume that Yuma was not a skilled duelist during the first and, perhaps, the second fusion (though he certainly became one during subsequent ones) but still good enough to manipulate possibilities into oblivion, dividing by zero and such, creating 100% from 0%, etc.etc

This goes very well with all the summons that you can play
Keilflamme-ex 098/071 UR Angriff: Hitzesto 60 Schadenspunkte
What size sleeves do your cards need