(From the anime/manga, Shingeki no Kyojin/Attack on Titan)
Giant titans that calls the walls of Paradis' walls a home, King Fritz used some Eldians and turned them into pure titans, 45 to 50 meters tall, 15 meters wide, and 5 meters across. They resemble Bertholdt's Colossus Titan, and releases massive heat.
These titans are weapons, used for world wide genocide. Only Eren (Main character of the story) has been able to use the full potential and start the Rumbling.
Armin: Wait, why does Eren need all of the wall titans in Shiganshina? Wait, he can't be commiting mass-genocide!
Eren: I'll use the wall titans to commit mass genocide and eradicate all life beyond the walls.