These weapons focus colossal amounts of energy at a point, to destroy a single target even at great distance.
The technology and form varies; but almost every major warring race in the 41st millennium uses Death Ray weapons of some sort.
When you make a Death Ray shot:
Pick any target within range and line of fire, even a specific unit in a detachment.
Roll a D6. If you roll a 2+ then the target suffers a hit, regardless of armour or cover.
A Death Ray hit on a War Engine unit always causes critical damage. Roll for a result on the unit’s Critical Damage table.