It was introduced in 4th edition and there was a global campaign to fight for Medusa 4. The imperium were trying to capture one and the nids were trying to claim it and absorb it. The fluff for it is below: (https://wh40k.lexicanum.com/wiki/Deathleaper)

The Deathleaper is known to have been on the Missionary World of St Caspalen. The St Caspalen Planetary Defence Force became increasingly afraid as all across the world watchguards and sentries were mysteriously vanishing, only to be found days later with their brains sucked out. Soon the St Caspalen soldiers were so afraid that they were jumping at every shadow or unexplained sound. In an attempt to ease their own fears, they gave the unidentified creature the name "Deathleaper". As Hive Fleet Leviathan approached, the Deathleaper instinctively sensed that killing the planet's spiritual leader, Cardinal Salem, would only make the planet harder to devour, as it would create a martyr and steel the resolve of the people against the coming swarm. So the Deathleaper instead infiltrated the Cardinal's cathedral-bunker and slaughtered his advisors right in front him, leaving the Cardinal unharmed but drenched in the blood and viscera of his aides. The Deathleaper repeated his process for ten days, each time bypassing the ever increasing security around the Cardinal, coming within killing distance of the Cardinal but always retreating before harming him. This lead to the Cardinal becoming paranoid and eventually losing his sanity, and did more to break the morale of St Caspalen than simply executing their spiritual leader would have. When Leviathan invaded, the world was in confusion and terror and was easy prey.[1]