Deck Heroes is a mobile trading card game that you can download from the Play Store. Thalassa is the first of the sealed 5 star cards and is the easiest to get. You need to have cleared level 4 and then use the explore option to collect shards for the creature cards that you will need in order to trade in for Thalassa. Then go to Remove Seal to unlock your card.
Thalassa has a special skill called recycle that unlocks at level 10. Recycle returns 1 card from your graveyard back into your deck. At level 10, Thalassa has an attack of 490 and health of 1675.