Infested siege tanks are Crucio siege tanks infested by the zerg. They retain their crucio shock cannon, but appear to have lost the functionality of their 90 mm twin cannons. They move around on legs similar to that of spine crawlers, and must root in creep to fire.
Infested siege tanks appear in the Nova Covert Ops mission Night Terrors. They are slower than their terran counterparts, and are unable to fire while not in siege mode. However, they still do heavy area of effect damage, and at the northernmost extractor the zerg will send in waves of infested siege tanks carried by (under) overlords, already in siege mode. These can do heavy damage to clumped defenses if not properly dealt with.
Alexei Stukov can construct infested siege tanks in Co-op Missions. They can attack the enemy with spine crawler, as well as consuming infested terrans, and shooting them out as projectiles in siege mode. These infested terrans become volatile infested, and can be controlled after they land nearby their target. Infested siege tanks heal for 20 health for every infested terran they consume.
Root: Roots the infested siege tank. A rooted infested siege tank has the ability to attack ground units with its violate cannon, but it's unable to move. Unroot: Uproots the infested siege tank. An uprooted infested siege tank has the ability to move, and it attacks with its impaler tentacle. Moves much faster on creep.