"Slithermuse" is a 3/3 elemental printed as a rare for the Magic the Gathering expansion set known as "Morningtide." From the looks of Slithermuse, you could put this in multicolored elemental deck or a blue control deck. Depending on how you use the Slithermuse, it could be somewhat useful.
The converted mana cost for Slithermuse is four. You will need two blue mana and two colorless mana to cast Slithermuse. For what Slithermuse can do, the converted mana cost seems to be pretty reasonable enough. Make sure that you use this in either a control deck or an elemental deck. With the right cards, Slithermuse can be pretty helpful.
What does Slithermuse do in the first place?
The first thing you need to know about Slithermuse is that it has an alternative Evoke cost. That costs one blue mana and three colorless mana. By paying the Evoke cost, it is sacrificed as soon as it enters the battlefield. So far, Slithermuse is one of those cards that is played and sacrificed simply for its ability. You would play Slithermuse to take advantage of the ability which activates upon leaving the battlefield.
When it leaves the battlefield, you get to choose an opponent. If s/he has more cards in his/her hand than you do, you get to draw cards equal to that different. Say that you have three cards and your opponent has six, you get to draw three cards. This is a handy ability to use and abuse if you are in serious need of cards.
One can ask: How do you get the use out of this ability?
One way would be using a black and blue control deck. You will need cards such as "Volrath's Stronghold" and/or "Mortuary." Volrath's Stronghold is a legendary land that allows you to pay a mana cost to bring a target creature card from your graveyard to the top of your library. Mortuary is a black enchantment that puts a creature card on the top of your library instead of your graveyard. You can use those abilities while paying the Evoke cost for Slithermuse.
This is a handy combination of abilities whenever you feel the need to draw more cards. In a sense, it is similar to using the Evoke ability of "Shriekmaw." Shriekmaw destroys a target creature upon entering the battlefield. In the case of Slithermuse, you get to draw cards.
Overall, this is a great card in a control deck. If you ever need to draw cards, you can use the main ability of the Slithermuse. In a green and blue deck, this can help out a card such as the "Lorescale Coatl" which receives +1/+1 counter every time you draw a card. Still, this is a good ability to exploit if and when you need cards in your hand.