Crown chest

Summary

The Dark Prince is unlockable from the Royal Arena (Arena 7).
He is a high hitpoint, melee troop that deals splash damage, making him effective against hordes of troops.
Alike the Prince, when the Dark Prince rushes, he will charge, gaining increasing speed. When the Dark Prince attacks a target, he will do 2x damage to the target and any nearby targets while immediately losing the speed bonus.
The Dark Prince can regain the movement speed bonus if he starts to move again.
Strategy

When fighting an opposing Dark Prince, an effective counter is to place a unit to withstand its charge attack, such as a Knight.
However, unlike the Prince, a high hitpoint troop is much more effective to counter the Dark Prince than deploying low hitpoint units to counter the card, as the Dark Prince deals area damage.
Excess damage from destroying the Dark Prince's shield will not affect his new hitpoints.
For instance, if the Dark Prince has 200 shield hitpoints, and a Lightning does 650 damage to him, the extra 450 damage will be completely negated.
The Dark Prince is effective at stopping the Prince's charge attack. Due to his shield having a separate health bar from himself, it will absorb the Prince's charging attack, keeping the Dark Prince alive, and allowing him to survive.
An attacking Dark Prince in affiliation with the Prince is immensely effective. The Dark Prince can eliminate opposing low hitpoint troops while the Prince charges towards the enemy Tower, unhindered.
Trivia

The Dark Prince was added to the game on 29/2/16 with the March Update.
On 23/3/16, a Balance Update decreased his charge speed by 13%.
Like its counterpart, the Dark Prince is not related to any other Supercell game.
The Dark Prince has notably less hitpoints than the Prince. However, the Dark Prince is protected by a shield.
Similar to the Knight, the Dark Prince's shield and mace fall to the ground when they are both respectively destroyed.
The Dark Prince's voice and battlecry are muffled. This is most likely because he wears a full-face helmet.
The Dark Prince and its counterpart ride the same breed of horse, as stated on Clash Royale's Official Website.
He seems to wear a chainmail as similar to the Knight's under his armor.
Dark Prince and Guards are the only 2 cards to have a shield along with their own health.
Dark Prince and Valkyrie are the only Melee troops that deal area damage.