The converted mana cost for Quagnoth is six. You are going to need one green mana and five colorless mana to play Quagnoth. For what Quagnoth can do, the casting cost is well worth it. You are paying six mana for a creature that brings forth three great abilities. These abilities work great against most opponents.
What does Quagmoth do in the first place?
The first ability is known as Split Second. Split Second is a good ability for a spell that is on the stack. By playing Quagmoth, opponents can only counter via mana abilities. In this respect, they cannot play spells that counter Quagmoth. In short, Split Second keeps opponents from being able to respond to Quagmoth being brought onto the battlefield. Even if your opponents could, Quagmoth has another ability.
The second ability is known as Shroud. It means that Quagmoth cannot be the target of spells or abilities. It affects you too since you cannot equip, enchant, or add counters to the Quagmoth. Still, Shroud ensures that opponents cannot do much to get rid of Quagmoth. The only two ways to get rid of Quagmoth would be to play a spell that destroys all creatures or causes you to sacrifice a creature. Also, Quagmoth can still get killed with combat damage.
The third ability allows you to return Quagmoth to your hand should some spell or ability force you to discard it. In this respect, Quagmoth can remain in your hand most of the time until you play it.
With these three abilities combined, Quagmoth is a great card to have in a green deck. There is the Split Second that reduces your opponents' options in responding to you playing Quagmoth. If they can use a mana ability, Quagmoth cannot be targeted because of Shroud. You see Split Second and Shroud working beautifully together. If you are forced to discard Quagmoth, you can bring it back to your hand. Overall, Quagmoth is quite the handy creature card to have. If you are playing an extended format with green cards, Quagmoth is a card you should have in your deck.