Summon is the most frequent and unique keyword for creatures and items in TES:Legends. If you play a card with keyword Summon the bonus effect of this card will trigger right away. Cards with all Attributes can have this keyword. Sometimes cards can have double Summon effect. This effect will trigger the second time after dying this creature( keyword Last Gasp). All Attributes in TES:Legends (Agility, Endurance, Willpower, Strength and Intelligence) and Classes (cards with Dual Attributes) have cards with Summon effect.
In Control decks creatures must have unique Summon bonus effects. You can play
In Midrange decks unique Summon effect is very important too. You can build a lot of combos. Unique Summon bonus effects of the cards
In Aggro decks you can also use unique Summon effects to deal more damage to your opponent. This is the main keyword in Arena.