Edge of Extinction
Edge of Extinction is one of 2 overpowered ranger skills used in SCs, particularly UW and DoA.
Every time a creature dies in spirit range of EoE, all creatures of the same species that are currently below 90% max health and in spirit range of the creature that died and also under the effects of EoE will take damage. When killing a large group of enemies, any that take more than 10% damage should die to all the EoE triggers, so it's important to deal off-damage.
The GWToolbox++ minimap can display circles (green by default) showing which areas are in range of EoE. Note that the circle is only visible if the spirit itself is in your compass range. The minimap also has a spirit range marker so you can see how far your EoE will reach before you place it. Texmod allows you to display the marker on the default GW compass. In most SC tactics, barring unusual mistakes, tanks will always pull enemies to the same locations, so EoE can be placed at the exact same positions every run, without needing to check the marker. Exceptions include Deathbringer and the first Foundry spawn (both in DoA) - these spawn in unpredictable positions, so you have to wait and see where they spawn before you know where to put EoE.
Being in range of 2 EoE spirits does not increase the damage - only the stronger EoE takes effect. If 2 creatures are each in range of a different EoE (and each other), the EoE that deals damage will be the one in range of the creature that died.