Demon Spider/Magiclysm
Magiclysm Demon spiders come in a few varieties but are all "weak" to bludgeoning damage and have complete immunity to fire. Their resistance to firearms is also quite great. The most common of which is the Demon Spiderling. These monsters are small with a health pool to match, but their hard exoskeleton and agility make them quite resilient. As with other spider monsters, these inflict poison, though their abnormally powerful attacks are likely to kill their prey before the poison does. These can be found in fields or in the Demon Spider nests in groups of 1-3. These can be butchered for a small amount of demon chitin.
The next larger variants of the Demon Spiderlings are the Demon Spiders. At this point, these spiders are an extreme threat to even the most well equipped survivor having access to a fire spell, powerful attacks, and massive resilience. Their resilience is great enough to make even commonly powerful 5.56 ammunition for weapons like the M4A1 do piddling amounts of damage or none at all. The upside to their increased size is that they are a bit slower. Unless your survivor is injured, you should be able to get away from these if you need. When butchered these can yield larger Demon Chitin Plates required for lategame armor or forge crafting. These are found in groups, almost exclusively in the Demon Spider nests.
Finally we have the strongest variant. The Queen. The Queen Demon spider will shrug off all but the strongest of ballistics attacks and has the ability to summon Demon Spiderlings endlessly with a fairly large delay. If you want to challenge this monster without magic, you'll need nothing short of high explosives or a lot of powerful, high caliber bullets such as .50BMG. With powerful armor such as the Demon Chitin armor to handle the heat and powerful skills, one may be able to chip away at its health in melee combat. While the queen is extremely resilient and powerful to match, she is very slow. If you manage to take down one of these goliaths, you can expect to acquire a lot of the normal mutant mean, but above that you'll have a massive supply of demon chitin. These can only be found in the Demon Spider nests where there will only be one.
The Demon Spider nests are filled with numerous valuable and powerful magic items. These nests are hidden on the overmap, labeled as "Forest" tiles but DO have a very distinct gray color instead of the normal green. They can be identified, in person, by their sturdy web walls and pockets of magma inside. These locations are dangerous, but potentially very worth the risk for a skilled survivor.