The Morklay Trap is a Second EraBuilding. It is essentially a bomb, that you can detonate, to send a powerful shock wave in the specified direction. It is very useful, when you are setting a trap for your enemy, and you know exactly were they are going to attack. That's also their main weakness in PvP, the enemy can decide to attack somewhere else when he sees it, or bring long range Siege.
One of the good areas to use it in, is in the Encounters with Twilight scenario. if you chose to defend and use the screaming stone to call the enemies to you, after all your towers are built, and units are in place, build a Morklay trap near your wall, and the activate the screaming stone.
Once the enemies come, you can detonate it, and possibly kill a large clump of enemies.
When combined with a Construction HutMorklay Traps offer an offensive PvP tactic. Once a Morklay is built in front of the enemy base, you can easily build another one or simply start to attack with your troops. Anything your opponent summons in early tiers will most likely die instantly along with doing a great of damage to your opponent's base.