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Everything was set for celebration. Banners with happy birthday wishes, little cupcakes with colourful icing. The only one missing was the girl they were celebrating. The whole clan stood before the entrance gate of ‘Their’ forest.
Four hours had already gone by, it was not the longest but also way longer than the average one hour of the other children in the last years. After the two hour mark hit, the other children were sent away to play having gotten bored of waiting and starting to cause mischief.
The girl's parents had worried frowns on their faces, the Nara clan head had a calculating look on his face.
Finally, nearly five hours after the girl had entered the forest they could see a small silhouette coming out of the dark forest. They were walking slowly, like every step was a challenge. Her parents stood up from the chairs they had sat down on nearly an hour ago.
The figure came closer, and the girl's mother let out a choked gasp when they were close enough to see the face.
Because out did not come the freshly five year old girl that was sent into the forest five hours ago. In her place stood a young boy.
He wore the same ceremonial robes the girl had worn, though they looked older and had small rips and stains. Twigs and leaves were stuck in his robes and hair, he also did not wear any shoes, and there were small cuts on his bare feet.
His steps were slow and he was stumbling every now and then. When he was near enough to let the gathered clan members see his face, silent tears began to roll down the face of the birthday girl's mother. Her husband tried to comfort her, though he too was near tears.
Because the eyes were empty, shadowy black holes in the small boy's face. Dark black veins were visible under the boy's pale skin.
When he arrived in front of the clan head, the child's face turned to Shikaku’s.
“The prize has been paid.” While the boy's mouth moved to the words, it was not the voice of a child that came out. It sounded like the boy had swallowed glass shards and was drowning in blood at the same time.
After the boy had ‘spoken’ he dropped to the ground, lifeless.
Shikaku crouched down, to check for the pulse that everyone knew wasn't there.
Then he put his arms under the lifeless body of the five year old and stood back up.
Turning to his clan he spoke “Our spirit has chosen a new host. May we honour Shikari's sacrifice and bury Shikaito’s body.”. The traditional line had been engraved in everyone's head for their whole lives. Though they had all hoped to never hear it.
Shikaku began walking to the girl's parents, so they may speak their last wishes for their child.
Nobody noticed the small boy watching the whole procedure with wide eyes. Shikamaru finally understood why his tou-san had been so tense the last months, why he played more with him, why he had untensed slightly when the boy came out of the forest.
His fifth birthday was next month after all.
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So, I imagine that the shadow jutsu of the Nara clan is something they get after meeting their spirit in their special ‘totally not connected to the spirit realm Hokage-sama, we don't know what you're talking about’ forest. The spirit needs a host, so about every fifty years one of the children sent in doesn't come back.
The reason the Nara are so lazy, is that most of their energy goes to their spirit. In return they can use the clan jutsu.
It's technically a kekkei genkai (bloodline limit) because a non Nara going into the forest will just disappear, (probably eaten by the spirits).
Those born into the Nara clan, their bodies have changed, so that the strain of being connected to their spirit doesn't really hurt them.
Because of all of this, their jutsu can't be copied, and bloodline theft is also pretty impossible because others don't have the special spirit forest.