Shadowshard Crystal is an Epiccrafting material in Save the World. They are needed to craft Tier 4 Shadowshard Weapons (Power Level 88-106 Weapons). You need a Tier 4 Shadowshard Weapon in order to evolve it into a Sunbeam Weapon (See Sunbeam Crystal for more details about the Tier 5 Crystal).
What Missions to Start
Shadowshard Crystals can be found in both Canny Valley and Twine Peaks. Shadowshard Crystals can only be found in Power Level 64-70 zones in Canny Valley and in all zones in Twine Peaks.
Shadowshard Crystals are also more rarer in Canny Valley as Tier 3 materials (Malachite Ore, etc.) are common in Canny Valley. They are more commoner as you advance through Twine Peaks with them being the most common to find in Power Level 88-100 missions.
How to Obtain
Shadowshard can be found in Caves and cliffsides just like ores. Breaking them gives 1-3 Shadowshard Crystals.
Shadowshard Crystals can be obtained by completing Expeditions. You usually get 8-11 Crystals from a successful Expedition.
Shadowshard Crystals can be obtained as a reward from destroying an Encampment. Look for a Encampment that will reward crystals upon destroying the encampment. Destroying the Encampment will reward either Quartz Crystal and Shadowshard Crystals. You will get 1-6 Quartz or Shadowshards.
Shadowshard Crystals can be obtained from the "Salvage the Drone" event. This event is random and only happens if you have an additional 1-3 players in your lobby. If it happens in your game, it is the best to go and defend the drone as you will get Shadowshard Crystals as a reward after successfully defending the Drone. You usually will receive 1-5 Shadowshard Crystals.
Be careful to not let the husks damage the drone as the more damage the drone takes, the less rewards you will get. If the husks destroy the drone before the defense ends, you will get nothing. It is best advised to have your team come and help defend the drone so that you will not struggle.
Shadowshard Crystals can sometimes be obtained by building a certain Radar Tower. Look for a Radar Tower that will reward crystals upon completing the radar. Completing the radar towers will give either Quartz Crystal or Shadowshard Crystals. You will get 1-7 Quartz or Shadowshards.
Shadowshard Crystals can sometimes be found from eliminating certain Husks and Mist Monsters. Minibosses from Miniboss Mission Alerts can also drop Shadowshard Crystals. Takers, Blasters and Smashers have a chance to drop 1-3.
Nurses and Lobbers also can have a chance to drop them. They usually drop 1-2.
Minibosses can drop 1-5 Shadowshard Crystals.
Shadowshard Crystals can be obtained from completing/winning a mission. They are mostly common in Eliminate and Collect/Resupply Missions, although they can appear in normal missions like Fight the Storm or Ride the Lightning. Resupply / Eliminate and Collect Missions can yield 1-4 Crystals while normal missions can yield up to 14 Crystals maximum per mission.
Shadowshard Crystals can be obtained from Storm Chests. Try aiming for killing all 50 husks and the mist monsters as at the end, it can reward many loot that are great including Shadowshard Crystals. Storm Chests usually give 1-5 crystals.
Storm Chests also can give you weapons and sometimes, that weapon can be a Shadowshard or Obsidian weapon. You can recycle that weapon and receive additional Shadowshard Crystals.