‘A "mulligan" is a decision made by a player to return his hand to his library for an opportunity to draw a new one after shuffling.’
The mulligan is the first choice that every player has to make in a game and it can seriously affect how your game progresses. Whilst it's possible to make a general list of scrolls that you probably wouldn't want to mulligan, the large variety of opposing decks that are being played means that there maybe a slight variation in what should be discarded first. With this in mind, this short article will give you some general tips on what you may want to keep.
The first mistake most beginners make is assuming that having high cost scrolls in their hand is the ideal. The problem with only having scrolls of, say, 4 or higher cost is that you have to wait for turn 4 before you can play them. What is more likely, you'll either need to draw a 1 - 3 cost scroll on subsequent turns or sacrifice one of your high cost scrolls for two different ones anyway. Whether you do the former or the latter, you'll need to sacrifice scrolls from your starting hand to generate resources to play any of them.
This leads to the concept of the Mana Curve. Each turn you will sacrifice a scroll to generate a resource point that allows you to play a scroll. You can only do this once per turn so, in theory, by turn 10 you would have created 10 resource points that allow you play one or more scrolls up to that value. On turn 1, you won't have enough resources to play a scroll that costs 4 resource points, so that scroll is only of value then. To gain the most advantage from the mana curve you would want to use up all of the points available to you each turn, so playing a 1 cost scroll on turn one, 2 cost scroll on turn two, and so on. There are other articles which probably explain this better. Draw - (get the right card off a random draw from your deck)
The second common mistake is when a player fills their hand with low cost scrolls that are 'situational'; scrolls which can be played in the early turns but don't impact the game as much as the player would like. One test common to MtG is the vanilla card test; a vanilla card is one which has no traits or abilities, so you simply look at its health, countdown, and attack to find its value. You'd assess its impact and survivability and, if the card looks weak, then it's most likely it's ability or trait that makes it have value. (kinfolk brave / sister of the fox) Further from this, if its ability is usable only in certain circumstances then it's value is decreased further. So, whilst you may want to have a lot of low value cards in your hand when the game starts, if they are easily removed from the game or have no use till certain conditions are met, then you'd be better off discarding those too. But what does this leave you with?
Every deck will have at least one scroll that would be perfect in your opening hand, and some players may mulligan their opening hand if they don't get that scroll straight away. Ideally, you would want to have multiple scrolls that you would like in your opening hand. Even better, you would want to have worked out various combinations of scrolls that you know can gain you board presence, remove threats, or simply gain you scroll or resource advantage. Whilst collectible card games aren’t chess, the same mentality needs to be applied in the sense that you know a specific combination of moves (read 'scrolls played') will have a specific effect.
An exception to the 2nd rule is when one of your high value cards might be vital to your strategy and you'd really like to be playing it as soon as you can. These are the cards you need to decide whether you should hold onto, or sacrifice in the mulligan and hope you get them at the right time. The cost of the scroll, and how many you have in your deck, can help with your decision if it’s necessary to keep or remove the scroll. If, say, the scroll costs 6 resource points (and you don’t have any scrolls which would allow you to draw it sooner) then you effectively have 6 turns to draw one of the 3 copies that you have in your deck.
Your primary goal as a player is to reduce the random aspects of a card game to the lowest degree and give yourself the most options that you can each turn. The mulligan can help you to play suitable scrolls from the very first turn and set yourself up to make those vital plays at the right time in the later game.
Sysp explains some of the nuances behind card sacrificing.
A detailed guide on the various ways players can use the black market to enhance their playing experience