A splicer that fuses Kyurem and a certain Pokémon. They are said to have been one in the beginning.
Description of DNA Splicers before fusing.
This legendary ice Pokémon waits for a hero to fill in the missing parts of its body with truth or ideals.
Description of Kyurem in B2W2.
It's only immoral and wrong when Ghetsis does it because he wants to use it to take over the world using White/ Black Kyurem. Ghetsis also uses Reshiram/ Zekrom against the Pokemon's wishes, and also against N's wishes (who is the Pokemon's unofficial owner), which means Ghetsis is forcefully doing this. Ultimately, N recognises the player as the new hero, giving them the light/ dark stone.
Kyurem, Reshiram and Zekrom all used to be the same Pokemon. When Reshiram became the truth seeker and Zekrom became the ideal seeker, Kyurem became what was left over. Kyurem fails to have the driving force like the other two dragons have, and it therefore can be fused with one of the other dragons to fill the void of what it's lacking.
You presumably can't fuse Reshiram and Zekrom together because there's only one in each game, in addition to truth/ ideals splitting up from the original dragon Pokemon in the beginning.