This card's effect that Special Summons it and another Level 8 Dragon monster from your hand is an Ignition Effect that activates in the hand. (It does not target. Revealing this card and another Level 8 Dragon monster in your hand is the cost to activate this effect.)[1]
This effect Special Summons both monsters from your hand simultaneously. The last part of the effect to resolve will be the 2 monsters being Special Summoned. (The part of the effect that prevents you from Normal or Special Summoning monsters other than LIGHT or DARK Dragon monsters for the rest of the turn is applied immediately once the effect resolves.)[1]
This card's effect that adds a Level 4 LIGHT or DARK Dragon monster from your GY to your hand is an Ignition Effect that activates in the GY. (It targets 1 Level 4 LIGHT Dragon monster or 1 Level 4 DARK Dragon monster in your GY. Banishing this card from your GY is the cost to activate this effect.)[1]
Q&A Rulings
Q: I activate the effect of "Nebula Dragon", revealing it and a "Galactic Spiral Dragon" in my hand. If my opponent chains "Ojama Trio" and Special Summons "Ojama Tokens" to my field so that I only have 1 available Main Monster Zone afterward, what happens?
A: The effect of "Nebula Dragon" Special Summons both revealed monsters at the same time. In this scenario, if you only have 1 available Main Monster Zone when the effect resolves, neither monster is Special Summoned and they both remain in your hand, but the part of the effect that prevents you from Normal or Special Summoning monsters other than LIGHT or DARK Dragon monsters for the rest of the turn is still applied.[2][3]