Edo Tensei is the one jutsu that managed to completely dictate and dominate the battlefield during the Fourth Great Shinobi World War. All the joint power of hundreds of Shinobi from all five hidden villages are completely halted in place through the sheer power of this incredible jutsu.
At the time, the one who uses Edo Tensei is none other than Kabuto Yakushi. However, Edo Tensei itself is not his original technique. It was created by the second Hokage, Tobirama Senju, but he quickly acknowledged the danger behind this jutsu and deemed it a forbidden technique. That is until Orochimaru got his hands on the secret of Edo Tensei and perfected it to its current state. So what can Edo Tensei actually do to make it deserve the title of The Most Dangerous Jutsu Ever Created?
Edo Tensei Can Raise Tons of High-Quality Soldiers
Edo Tensei is a forbidden jutsu that enables the caster to raise people from the dead and do the caster's bidding. No matter who they are or how long they've been dead for, this jutsu can still forcefully summon their soul from the Pure World and give them a vessel that allows them to exist in this world of the living, also known as the Impure World.
The main difference between Edo Tensei and other kinds of resurrection techniques is the fact that the summoned soul is not actually turned into a living being. They will be able to move and even converse just like the time when they were still alive. Their body, on the other hand, is akin to that of a zombie. So in a way, the summoned soul turns into an undead.
Immortal Soldiers With Unlimited Chakra and Stamina
As mentioned before, the dead that are summoned by Edo Tensei possess a unique kind of body. Appearance-wise, they look pretty much identical to how they were when they were still alive. However, since they are already dead, the summoned people cannot be killed anymore. Not only that, any damage to their new body will quickly be repaired. Even missing limbs will also be regenerated as well. Needless to say, they also don't get tired either.
That being said, the best thing about Edo Tensei is the fact that the undead can use the same power as the ones that they wield when they were still alive. No matter how bizarre or unique their abilities are, they can be freely used by the undead under the direction of the caster. The combination of a true undead body and the unlimited supply of chakra and stamina makes these summoned undead fight for eternity.
Total Control of The Entire Undead Army
Resurrecting dead people and giving them their original abilities back, along with an immortal body is just one part of the equation for Edo Tensei. The other part is controlling those resurrected people like a puppet. Upon resurrection, the caster can put certain commands onto the body of the dead to make them submit to the caster and follow everything they say.
The caster can leave the personality, memory, and free will of the dead intact. Kabuto uses this strategy as some sort of psychological attack. After all, everybody would certainly feel hesitant if they suddenly had to face their deceased loved ones. However, the caster can also override their personality and take control of their body by force. In that sense, the summoned soul has truly become a mere puppet and turned the caster into a one-man army.
The Ritual for Edo Tensei is Relatively Easy to Do
For such a dangerous and versatile jutsu, Edo Tensei itself is relatively easy to do. There are only a handful of things that need to be prepared in order for it to work properly. The first one is definitely the knowledge regarding the Edo Tensei itself, such as the hand seal, the written command, etc. Orochimaru has perfected the jutsu, which is how Kabuto knows how to use Edo Tensei in the first place.
The second thing that needs to be prepared is a living human sacrifice. More often than not, the original body of the dead individual is no longer in a usable state. Which is why the caster needs to prepare a new, living, preferably healthy body, for the soul to reside.
Lastly, the caster needs to have the DNA of the individual that he wishes to summon from the dead. It can be their whole body, or simply just blood, or hair. If the caster cannot find the original body of the deceased, or couldn't harvest their DNA, then the resurrection wouldn't be possible, which is the reason why Jiraiya can't be summoned through Edo Tensei. After all, his body is now in the bottom of the ocean after losing against Pain.
In theory, everybody can be resurrected with Edo Tensei, but in practice, there's a caveat to this technique. In order to be properly resurrected, the soul must reside within the Pure World of the dead. This is why every soul that has been sealed or has been used as a medium to seal others, such as every previous Hokage, can't be summoned with Edo Tensei. At least not until their souls are unsealed.
Edo Tensei Has No Risk For The Caster
With great power comes great risk to its user. Izanagi, the power to rewrite reality itself is arguably sharingan's greatest technique. However, once Izanagi is used, the eyes of the user will be forever blind. Even Naruto's Rasenshuriken that can penetrate all the way to the cells of the opponents also bears a heavy cost to its users. Upon prolonged use, Rasenshuriken will inevitably damage the nerves on Naruto's hands and render it useless.
That being said, Edo Tensei does not have such a steep price to power ratio. As Kabuto explained it to Obito Uchiha, Edo Tensei doesn't bear any risk whatsoever to the caster, because it is essentially a summoning technique. But rather than summoning a giant toad, you summon the soul of the dead instead.
You can use Edo Tensei to resurrect however many individuals from the dead as you want, and there won't be any repercussions to you as the users. All of those abilities combined with this no-risk nature of the technique are the reasons why Edo Tensei is the most dangerous jutsu in Naruto.