FLIP: You can target 1 monster on the field; change it to face-up Attack Position if it is in Defense Position, also change its ATK to 0. You can only use this effect of "Subterror Behemoth Speleogeist" 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.
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.
Black Rose Dragon has a fairly obscure second effect compared to her field nuke effect. She does the same thing of changing the opponent's defense position monster to attack position and making their attack stat 0. Although it is unlikely you will use Black Rose Dragon in a Subterror deck, this can be useful when you want to use Subterror Behemoth Speleogeist's effect outside of a Subterror deck.
That's actually a very strong effect considering the many ways to force opponent's monsters into Defense Position.
Note that it doesn't care whether the target is already face-up or not, it simply needs to target a monster in Defense Position. Meaning you can Canadia/FTH an opponent's monster then later on target that same monster with this effect.
This card is actually broken, I used mausoleum of the emperor to summon a dark magician and used anti magic arrows before battling to take care of my enemy’s back row and when this monster flipped face up it’s effect activated. However it targeted my dark magician (which is in attack position) and set its attack to zero after destroying it.