Thallid Shell-Dweller has at least three spore counters on it.
Vorel of the Hull Clade does not have summoning sickness.
Creatures you control can tap to produce at least ({G} magic symbol) ({U} magic symbol) .
Steps
Activate all mana-producing creatures you control by tapping them, adding at least ({G} magic symbol) ({U} magic symbol) .
Activate Vorel of the Hull Clade by paying ({G} magic symbol) ({U} magic symbol) and tapping it, doubling the number of counters on Thallid Shell-Dweller.
Activate Thallid Shell-Dweller by removing three spore counters from it, creating a 1/1 Saproling creature token.
When the Saproling enters the battlefield, Intruder Alarm triggers, untapping all creatures.
Repeat.
If creatures you control can tap to produce at least ({1} magic symbol) ({G} magic symbol) ({U} magic symbol) , this combo nets infinite mana and you may target any creature using Vorel of the Hull Clade.
This combo also nets infinite spore counters on Thallid Shell-Dweller if it has four or more spore counters to start the combo.
Results
Infinite creature tokens.
Infinite ETB.
Infinite mana creatures you control can produce.
Infinite proliferate for artifacts, creatures, and lands.