What is Excavate in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Excavate keyword.
Excavate is a keyword added to Showdown in the Badlands (alongside Quickdraw). Excavate cards give you a treasure from a curated pool based on your tier of excavating. Your first use of Excavate will give you a Common 1-mana treasure (despite the presence of a rarity gem, the treasures are not collectable by any means. They are merely a visual indicator to designate what "tier" of excavating the treasure belongs to).
All classes have access to some level of Excavate, but it's used as the main mechanic for classes that belong to the Bloodrock Mining Company (Death Knight, Mage, Rogue, Warlock, and Warrior). These classes each have a Legendary treasure that they can access once they reach the Legendary "tier" of treasures, whereas the other classes will only be able to access Common, Rare, and Epic treasures. After a class reaches their highest "tier" of treasures, their next Excavate card will simply put them back at Common.
Keywords That Work Great with Excavate
Excavate is a keyword that is based on surviving long enough to play all your Excavation cards. Naturally, keywords that help you stabilize and stay alive on the board like Taunt, Rush, or Lifesteal will work towards your goal of staying alive.
Beyond that, Dredge can be used to possibly fish up an Excavate card at the bottom of your deck, and Discover can possibly generate extra copies of Excavate cards.
Playing Against Excavate in Hearthstone
The biggest part of playing against Excavate cards is to take note of the class you're playing against. If they aren't part of the Bloodrock Mining Company, then you won't have to worry about them generating Legendary treasures at all, so you can immediately scratch that off. If you are playing against one of those classes though, do take note of the fact that there are only 5 Legendary treasures, meaning that each one has a 20% chance of being chosen. You should try to devise a flexible plan for dealing with as many as you can at one specific time when your opponent generates one.
Since Excavate "tiers" scale in the same linear fashion each time, you will be able to track what "tier" your opponent is at even if they don't necessarily play the treasures right away by simply keeping track of how many Excavate cards they've played this game. Against Bloodrock Mining Company classes, every fourth Excavation will generate a Legendary. Against other classes, every third Excavation will generate an Epic.
Most of the Common and Rare treasures are ones that you won't need to play around the possibility of your opponent having, although there are a couple of exceptions (namely, Canary at Rare tier). All of the Epic treasures however are quite strong and are powerful in many different ways that it's quite difficult to play around all of them at once. Collapse can be a devastating AoE against a board of small minions, and Steelhide Mole can be a tough and annoying wall to break through. All of the Azerite treasures are also quite unpredictable because of their wide Discover spells, so watch out if your opponent plays one of those. Classes that exceed at spellslinging can also get really mileage out of Pouch of Coins since it effectively works as 3 triggers for spellcasting effects for -1 mana, or a means of play extenders. Rogue can also use Legendary treasures for Burgle synergy since each one will count as a certain class instead of all the Common, Rare, and Epic treasures which count as Neutral.
Example Hearthstone Excavate Cards
Reap What You Sow is a simple damage spell for Death Knight that Excavates and deals 3 damage. It does require one Frost Rune meaning that Triple-Blood or Triple-Unholy decks won't be able to run it (but may still generate it), but it leaves the door open to mixing runes.
Blastmage Miner is a Mage card that Excavates and also has an Arcane Missiles-type of effect that scales for each card you're holding. Naturally, this works with the fact that the Excavate will add a treasure to your hand.
Harrowing Ox is a Death Knight card that gives you a bonus effect if you've Excavated at least twice during the game by making your next card this turn cost 7 less mana to play. In most situations, this will simply give you a card for free, but even if you do have to still pay mana for it, you will always be able to play that card once you've reached 10 Mana. It requires one Frost Rune to play, which compliments design with Reap What You Sow.