FLIP: You can change all other face-up monsters on the field to face-down Defense Position. You can only use this effect of "Subterror Behemoth Ultramafus" once per turn. When a face-up monster you control is flipped face-down, if you control no face-up monsters: You can Special Summon this card from your hand in Defense Position. Once per turn: You can change this card to face-down Defense Position.
Flip your opponent's mosnters face-down so they cannot activate their effects.
You can opt not to activate this monster's effect because it will change your monsters to face-down. Another option is to flip this monster first before flipping other Subterror monsters.
All Subterror Behemoth monster have the shared Special Summoning condition of “When a face-up monster you control is flipped face-down, if you control no face-up monsters: You can Special Summon this card from your hand in Defense Position.” and the effect of flipping themselves face-down, so once you get one Subterror Behemoth on the field it would be easy to get more on the field. The problem is that all Subterror Behemoths are high-leveled monsters. Some like Subterror Behemoth Dragossuary and Subterror Behemoth Stygokraken can be summoned with only 1 tribute to get Subterror summonings going. You can also use Subterror Nemesis Warrior to Special Summon Subterror monsters.
This is the best support Subterrors have. The first and second effects hasten the activation of your Subterror’s effects. The third effect gives your monster a massive boost in stats, which could be enough to OTK your opponent. The fourth effect provides protection to your Subterrors while they activate their effects. All while being reusable because it sets itself back to your backrow after use.
The downside is you can only choose one of these effects per activation. But you can set multiple copies of this card and use all of them every turn.
Other Flip Effects
Here are some other monsters with good Flip effects and are able to flip themselves back down. Subterror Behemoth Ultramafus can also help these monsters reuse their effects.