Egg Serpentleaf is the boss of Sylvania Castle Zone in Sonic the Hedgehog 4: Episode II. It is a giant mech that looks like a snake combined with a flower and is piloted by Dr. Eggman. It can transform into a flower and it has a gun below the Egg Mobile. The Egg Mobile itself is set on the dome of the mechanic plant and is the only place, where the player is able to damage it eight times.
Before the boss battle starts, Sonic and Tails find Dr. Eggman riding in his Egg Mobile, but he instantly flees. When they catch up to him again, Eggman plays a little trick on the heroes before revealing large totem poles, which later get suddenly destroyed by Egg Serpentleaf's mechanical tentacles.
When the battle first begins, the tentacles will occasionally swing across the ground to try to hurt Sonic and Tails. When it takes four (console) or three (mobile) hits, it will raise up two more tentacles, and Dr. Eggman will start shielding himself with the flower petals more. Otherwise, it is the same as earlier.
Finally, when he takes three (console) or two (mobile) more hits, Dr. Eggman will shield himself with the flower, but this time, the tentacles wrap around the stem, inserting some type of liquid into it, and the bottom of the Egg Mobile will reveal a cannon. If Dr. Eggman is left undamaged for a long enough time, the weapon from the bottom of the Egg Mobile with shoot out fire that will cover the entire ground. But with two (console) or one (mobile) more hits, the boss is defeated, and Sonic and Tails will hit the Egg Capsule, ending the Boss Act.
- There is a reference to Sonic the Hedgehog 2 within this boss, as before the battle starts, Dr. Eggman raises the stone pillars from his boss battle in Aquatic Ruin Zone, tricking the player into thinking they would fight a repeat of said boss battle, only for the Egg Serpentleaf to reveal itself and destroy the pillars.
- This also references the fact that in the previous episode, it only had reused bosses at the end of each Zone.
- The music that plays for this battle is the same as the background music as boss pinch mode and E.G.G. Station Zone in Sonic the Hedgehog 4: Episode I.
- This is the only Eggman mech boss in Sonic the Hedgehog 4: Episode II that doesn't require the Rolling Combo to beat.
- The Egg Serpentleaf looks to be inspired by Biollante's Flower Beast Form from Godzilla vs. Biollante, being a gigantic flower with mouthed snake-like tentacles that it uses in combat.