Effect
This effect may occur once every 60 seconds.
Unstable Anomaly
Unstable Anomaly is a defensive passive that saves your life in an emergency: if you would take lethal damage, you don't die but instead gain a strong shield for a short time that absorbs a multiple of your maximum life. At the same time you unleash a shockwave that knocks back and stuns enemies, buying you precious seconds to reposition or escape.
Because the effect can only trigger once per minute, the passive works like a cheat-death mechanic and is especially valuable at high difficulties or while pushing, where a single burst would otherwise kill you. A passive slot is most worthwhile for squishy glass-cannon builds with little defense of their own. If you already play very tanky or rely on constant damage avoidance, you can hand the slot to more offensive passives instead.
The seconds after it triggers
The shield lasts 5 seconds and absorbs damage equal to 400% of your maximum Life, and the shockwave also stuns enemies for 3 seconds. That window is not a free pass but an invitation to retreat: use it to vanish from the danger zone with Teleport or to step out of whatever caused the hit, such as an affix effect on the ground. If you simply stand still, you may burn through the shield without changing your situation at all.
The minute afterwards
Once it has triggered, you spend 60 seconds without this safety net. During that time more careful play pays off: take fewer positioning risks and use your protective skills deliberately. If the passive fires more often than you'd like, work on toughness and avoidance rather than leaning on the lifeline.
Unstable Anomaly and Galvanizing Ward
Galvanizing Ward is the other common protective passive: it helps before things get dicey, whereas Unstable Anomaly only steps in at the very last moment. Audacity rewards damage at close range, exactly where lethal hits are most likely. Storm Armor, Magic Weapon and Arcane Torrent make up the offensive frame the passive protects.
Unstable Anomaly only unlocks at level 60, so it arrives late while leveling.