In Eternal, as in many online card games, the choice of cards you can play during your turn is constrained by your Power level. Power in Eternal doesn't increase automatically but must be obtained by the player. Balancing Power vs other cards is a major part of deck building strategy.
How To Get Power
You start with a maximum power of zero, the main way to gain more is to play power cards from your hand. You can play no more than one of these per turn.
The 'standard' power card is a Sigil. This increases your maximum power by one and gives you one additional point of influence in the relevant faction. You're assumed to have an infinite number of Sigils and there's no maximum limit to how many you can place in your deck.
There are also other cards - Seats and Banners - that give you one additional maximum power along with influence in two factions. These cards normally have a precondition for playing them - for example you must have a Sigil in your hand or a unit on the battlefield. Seats and Banners must be obtained or crafted like any other cards and are subject to the usual 'four per deck' restriction.
Other ways to increase your maximum power include cards such as Initiate Of The Sands.
Famine & Flood
Since power cards must be drawn from the deck before being played it's possible to have games where the RNG is against you and you either don't draw enough power cards or draw too many of them. These situations are known as Power Famine and Power Flood, similar to Mana Famine and Mana Flood in other games.
So what can you do about this? If your initial hand looks bad you can always Mulligan and redraw. That way you're guaranteed at least two and no more than five power cards. But remember that Seats and Banners count in this number. If your Mulligan redraw gives you only these and no Sigils then basically you're screwed.
Other than that all you can do is balance your deck and hope for the best. Cards which give you extra draws - for example Dark Wisp or Ancient Lore - give you more chances to beat the RNG. There are also cards such as Find The Way and Celestial Omen which give you more control over which card you draw.
This page written for Eternal version 1.17, Card Set 1
This page last updated: 1-03-2017