20Shimmer
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Throwing Flame Trap into Shimmer transmutes it into Spiky Ball Trap. Obtained by shimmering: 
Super Dart Trap.All transmutationsharvesting one
Do not count on finding many. A temple can contain a single one, or none at all, so if you are collecting traps for a farm you may need to visit more than one world. Getting it out of the wall means a Picksaw, or explosives once Golem is down.
using it in a farm
It is the strongest of the temple traps for a wired kill room. One pulse is not one shot: it lets off a rapid burst over about a second, and the jet has no pierce limit, so a single activation cuts through an entire packed corridor. The reset between activations is long, so build several on staggered timers rather than one on a fast clock.
Two placement details save rebuilding later. The fire passes through one solid block, so you can bury the trap behind the wall of your corridor and keep it out of the way. But water, honey and shimmer all stop the jet dead where lava does not, so a farm that floods its floor to slow enemies down will also blunt its own traps.
on the receiving end
Walking into one inside the temple hurts badly and leaves you burning for a long time afterward. On the harder difficulties it is one of the few environmental hazards that can end an unprepared run outright, so watch the floor and ceiling on the way in.