Gigantic void capable xenos warp artefacts of the Yu'vath, Whisperers are seemingly autonomous and some have survived the alleged destruction of the Yu'vath. In appearance, they are a massive ball of dark, reflective energy several kilometres across. Around it orbits a collection of monolithic rounded crystals, power constantly arcing between them. They are known to have downed imperial cruisers with their direct attacks but their true danger lies in their system wide mind controlling abilities.
Given how the Warp works, faith is quite literally humanity’s only way of fighting this sort of abomination. Don't listen to him, he and the rest of them are clearly mind controlled by the whisperer. The real trick is to stop mumbling nonsense and to start firing those macrocannons