Zekrom resembles a large mechanical-seeming dragon, it is bipedal and mostly black/dark gray in coloration. Parts of its body regularly glow blue as electricity is generated. It has a bulky appearance, in contrast to the appearance of Reshiram. Its most notable feature if the large generator embedded in its tail. On the back of its head is a strange crest that may be based on thunderclouds.
Zekrom is one of the Legendary Pokémon spoken of in the Unova region's legends. Originally it was part of a single dragon along with Reshiram but split into the two dragons. It and Reshiram apparently share history with Kyurem. Zekrom represents Black Yin and is a Pokémon that prefers strong ideals.
When not resting in the form of the Dark Stone, Zekrom can be found flying the world's skies, wreathed in black clouds. During the course of Pokémon Black and Pokémon Black 2, it can be found at the top of Dragonspiral Tower. During the events of Pokémon Black, Zekrom can be battled as an opponent but not legitimately caught. The reverse is true in Pokémon White.
This Pokémon appears in legends. In its tail, it has a giant generator that creates electricity.
In Pokémon Black, Zekrom is first seen at the top of Dragonspiral Tower, having been awoken by N. It is later fought at N's Castle. It later is seen leaving with N to a distant land. It cannot be captured legitimately in this game.
In Pokémon White, Zekrom at first is in the form of the Dark Stone and must be awoken by the player, doing so at N's Castle, and can be caught before facing N and Reshiram in battle. If the player has a full PC and party then Zekrom must instead be beaten in battle; it can later be found at the top of Dragonspiral Tower, available for capture.
In Pokémon Black 2, N first appears with Zekrom during the battle against Ghetsis and Kyurem. After a lengthy cutscene, Kyruem absorbs Zekrom and becomes Black Kyurem - which the player must then fight. After the battle, Zekrom appears as N's ally once again and must be battled. It then reverts to its Dark Stone form, and will be available for capture once the player brings it to Dragonspiral Tower.
In Pokémon White 2, Zekrom does not appear, making it the only Generation Vgame in which Zekrom does not appear. It was caught by the player of Pokémon White, chronologically, who also does not appear in this game.