These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Naferia's Reign: Invasion of the Dark Mistress
Demonic Spiders / Goddamned Bats: Basically, any enemy can potentially fall under this with the right [or wrong] conditions, usually dependant on how high the Difficulty Levels are set, or if they pick up specfic items before you can kill them off.
Any enemy that awakens with or grabs an "Icon of The Defender" is instantly at the top position on this list. Not only can they one-hit kill you (usually, unless mortal-hit survivability kicks in) by just getting up within arms distance, but if you shoot them and aren't invulnerable yourself, you'll receive a nasty shock as all of the damage the invulnerable creature would've taken is dealt back to you instead, and this has caused many a player to get their current character killed by just deflected damage alone.
The next highest position on this list is the monster having either a "Regeneration Ankh" or a "Shield Emblem", or both of them at once. These items are Regeneration powerups for Health/HP and Armor/AP respectively. When these values are under 100% for a player or monster, each recovery interval is +15% HP/AP, and it won't stop at 100% HP/AP, and will continue to climb by +5% HP/AP per interval when HP/AP is above 100%, up to the true maximum of 200% HP/AP. If the monster in question has good enough defense, it can drain a ton of ammo you wouldn't expect to expend to kill them off.
Next on the list is probably any enemy with a Quad Damage, paired with any of the aforementioned artifacts or the Vampire Mask below. The Quad Damage is exactly the same as the one in Quake 1 & 2, allowing all attacks by the user to deal quadruple damage, complete with the guitar riff when an attack is executed. This is nasty when the monster's attack power is already high enough to wreck havoc (usually in the higher-difficulties) without it, leading to frustration from many a player when this happens.
Probably the lowest ranking powerup on this list is the Vampire Mask, which allows you to drain a small amount of HP/AP damage done to your enemies to your own HP/AP to heal those statistics. However if combined with the Quad Damage or the regenerative powerups above, it can make an enemy much harder to kill.
Although a rare monster who somehow wakes up or attains all five of these is easily the most Demonic Spider worthy on the list.