The stalkers are an upcomming/discussed mob for the nether enhanced. being a non direct damage mob, and just to keep you on your toes.
They are a neutral territorial mob, meaning that if you stay 1 block away from this mob (being near the mob), or damaging it, it will follow you to the overworld causing some visual effects.
It is rare, but not too much
These effects are what make it the mob, having a five minute cooldown beetween each, and randomly choosing one, the effects can last from 5 seconds to 1 minute, and the cooldown is where the stalker is visible, but it usually teleports to the darkest area (AKA: cave systems and outside of your house), in this form, they will look at you directly.
Mobs with a slightly darker colour and with red eyes, they will follow you, once they get to the damage area, they dissapear.
any block near you and an stalker (which would be invisible at the time) will change randomly with another one nearby, this includes air blocks.
Those effects can make it one of the most complex mobs on the mod, but they are all just visual
This mob, has around 20 health (same as you) and a slow moving speed, it can drop gunpowder, bones, and darkness leather when killed.
It can only be killed with gold-tier tools, or with any tool at direct sunlight, killing him with non-gold tools at a dark place will cause him to "eat" the tool, reducing its durability but it wont damage him, if you kill him with any eaten tool it will give back the lost durability (gold wont count) or the whole tool, assuming you wasted your tool on him.