The eight types of Krana are sentient, organic beings that provide the guiding intelligence as well as a set of special powers to the Bohrok swarms, so the BBTS expansion implements them as monsters that allow Bohrok access to some effects that are generally a bit more clever than what the archetype otherwise does. Each Krana essentially has two effects: One that activates in the hand and is different for each monster while following one of two templates, and another that can only be one of two options, each of which is shared by half the Krana.
Let’s just use those second effects to segment the Krana in the following explanation and cover the rest as we go.
Based off the idea that Bohrok are really just robot suits piloted by the Krana inside them, these monsters have the ability to return themselves from the field to the hand to get any of the Level 4 Bohrok directly from the Deck. This is not something you want to rely on unless you need to since it only works during your Main Phase 1 and then ends the turn, but having the option at least avoids total bricks when playing a lot of Krana.
Regarding the first effect, the Krana Yo and Krana Ca follow the template of equipping themselves to a Level 4 or higher Bohrok from the hand in order to grant some continuous benefit. The Yo, holding the power to let its Bohrok dig through most substances, allows the equipped monster to attack directly. The Ca with its shielding powers protects all of your Bohrok from battle, but only once per turn for each.
Krana Xa and Krana Ja feature the alternative effect template, which means you can activate them by sending them directly from the hand to the GY at a certain timing. With the Xa, which are mainly in charge of formulating the more complex strategies of the plans, you can counteract negations or other responses to your Bohrok effects. It can be activated in any chain that has a Bohrok monster’s effect anywhere but as the last link and will negate the effects of all non-Bohrok cards on the field that appear in this chain. This lasts all the way to the end of the turn, so you might be able to disable something vital using this if you can just bait an activation out of it first. Another mass effect negation option is provided by the Krana Ja, which gives the swarms advance warning of known threats and thus renders everything that is visible on your opponent’s field useless during the following turn. This effect can be activated at any time as long as you control a face-up Bohrok, so you’d probably want to use it during the End Phase for maximum effect.
The other group of Krana are those that can banish themselves from the GY to steal a monster destroyed by a Bohrok from your opponent’s GY, controlling their enemies to make them part of the swarm. The monsters are Special Summoned in face-down Defense Position for thematic reasons and there’s a restriction on it that effectively prevents you from doing it more than once per turn, but even with that it’s obviously a damn strong move.
The equipping Krana among these are the Krana Za and Krana Su. The squad leader-type Za allow Bohrok to communicate and coordinate telepathically, which in this case is used to protect the equipped monster via strategic retreats of your other monsters (including face-down ones) while also keeping up the card supply. The Su is the caveman among the Krana, merely granting a stat boost, but that is very versatile in its simplicity, especially considering the equipping is a Quick Effect.
The pure hand effects here belong to Krana Vu and Krana Bo. The Vu makes Bohrok capable of flight, so it can be used to dodge targeting effects and even goes as far as negating and destroying the card in question to really make your opponents think twice about targeting your Bohrok with anything. The Bo can be triggered in response to your Bohrok cleaning up any card with their removal effects, using its night vision capabilites to track down further copies of the same card hiding in the darkness of your opponent’s hand and get rid of them for good.
On the topic of Krana that can steal your opponent’s monsters, we should take another look at Bohrok Confrontation, which was already covered in the main Bohrok article. What it does is send a Krana from your Deck to the GY to boost a Bohrok’s ATK/DEF, and knowing what we do about Krana now, the idea is obviously to have a Bohrok run over a monster in battle and then immediately steal it with the Krana.
There are other cards that further expand on the idea of Krana mind control. The following four monsters, for which I like to use the umbrella term “Servants of the Swarm”, represent the unwilling victims of this power.
Bohrok Servant is the generic standin for arbitrary beings under the control of a Krana. Its purpose is basically to make immediate use of the face-down monsters you get from the stealing effect, by contact fusing them (remember, that works when face-down) with a Krana from pretty much anywhere, banishing both and giving you a monster that at least copies the stats of whatever you stole.
The remaining Servants of the Swarm are a bit more specific, each of them being based on one particular inhabitant of Le-Koro that was possessed by a Krana during the takeover of that village. They generally work by sending a Krana from the Deck to the GY to neutralize an opponent’s monster and set up a situation where you can easily steal it, though the way in which they do so differs greatly.
Matoran of the Swarm counters Xyz Monsters by attaching to them as a material, which locks their effects and makes their ATK/DEF become 0 during battle with a Bohrok. It can also attach itself from the GY to your Xyz Monsters that are flipped face-up, obviously so a stolen Xyz can actually have material.
Turaga of the Swarm interferes with Synchro Summons in a particularly funny way, inserting itself on the opponent’s field in place of a Tuner and immediately forcing a Synchro Summon. When used as material, it gives you control of the Summoned monster for a turn and permanently makes it so its ATK/DEF become 0 when battling a Bohrok. This approach comes from the fact that Turaga were originally Tuners, so now that the BCOT remake is changing them to Link Monsters this card might be due for a redesign sooner or later as well.
Toa of the Swarm is the simplest of these, since it just Tributes over an opponent’s high-level monster and then surrenders itself willingly by making its ATK/DEF 0 when battling a Bohrok. The Level restriction was already iffy to begin with and is even more so when considering the prevalence of Link Monsters, so I might eventually tweak that to something like “2000 or more ATK”.
Finally ,we have a funny little card that simultaneously supports and counters Bohrok/Krana decks. Krana Pit lets you protect a card from destruction each turn by banishing a Krana monster from the GY instead, but since this works on both GYs you can just as well use it if your opponent is the one playing Krana. Similarly, it allows recovering a banished monster when there are 2 or more Krana banished, without specifying whose Krana they must be.
Conclusion
Krana add quite a few sophisticated effects to the Bohrok toolbox, giving you interesting options to interact with your opponent beyond just razing their field to the ground. The ability to steal a destroyed monster makes the Bohrok’s effects and attacks a bit more threatening than they already were, and with the Servants of the Swarm can itself be made into a central strategic element of your deck.
A selection of Krana can be found sprinkled throughout the sample Bohrok decks in the BBTS release, while the Servants of the Swarm usually appear as Side Deck options.