You can send 1 face-up "Umi" you control to the GY; destroy all other cards on the field.
Rulings
Individual Card FAQs
If you activate "Levia-Dragon Daedalus"' effect and your opponent chains "Ring of Destruction" to destroy "Levia-Dragon Daedalus", "Levia-Dragon Daedalus"' effect still resolves even though it has been destroyed.
You can send "A Legendary Ocean" to the Graveyard to activate "Levia-Dragon Daedalus"' effect, but you can't send "Maiden of the Aqua".
You can't send a face-down "Umi" or "A Legendary Ocean" to activate "Levia-Dragon Daedalus"' effect.
When "Book of Moon" is chained to "Levia-Dragon Daedalus"' effect to flip it face-down, the effect still resolves, but "Levia-Dragon Daedalus" is destroyed by its own effect, since it's no longer "this card", it's just "a card".
Netrep Rulings
Sending “Umi” to the Graveyard is the Cost of activating the effect of “Levia-Dragon - Daedalus.”
WATER|
Level 7|[ Sea Serpent / Effect ]
ATK 2600|
DEF 1500