Ninja Crusaders (忍者クルセイダーズ 龍牙?, lit. "Dragon Fang Ninja Crusaders") is a Nintendo Entertainment System game that is in the style of the original Ninja Gaiden trilogy of games for the same system where ninjas have to fight against aliens.
One day, a force of alien invaders wreaked havoc on our planet Earth from the furthest reaches of outer space, wreaking havoc on its inhabitants and killing most of them. This new menace was stronger than out most powerful defenses and proved to be unstoppable. In Japan, brave ninja warriors rose from the shadows to battle the alien invaders, but were eventually driven out as the menace took control of their homeland. Some of the ninjas escaped Japan and scattered to remote lands around the world. But their passion to reclaim their land and crush the enemy's rage. It was decided that two ninjas, Talon and Blade, would be sent to destroy the evil.
The game is a classic platforming ninja action video game that is similar to Ninja Gaiden. However, the game has its own unique feature. There are four animals the players can morph into by holding up or down + attack button: a tiger (Genbu), which is the fastest and can jump the highest but has the shortest range; a falcon (Suzaku), which has the ability to fly above everything but has no weapon at all; a scorpion (Hiryu), which for some reason can swim better than all the rest but moves very slowly on land; and finally a dragon (Seiryu), which can fly, is invincible, and kills everything in one hit, but the player can only use him for a limited amount of time.
Source: Wikipedia, "Ninja Crusaders", available under the CC-BY-SA License.