Mods/Thaumcraft/Mobs

Thaumcraft offers several new mobs:

Angry and Furious Zombies
Angry zombies are simply larger, more aggressive zombies with red eyes. They spawn occasionally among ordinary zombie spawns, but more often in magical biomes, notably Eerie biomes. Along with the usual drops, they frequently drop Zombie Brains, which are useful for some advanced crafting. There are also Baby Angry Zombies and Angry Villager Zombies.

Furious Zombies are even tougher, and actually grow larger when they are hurt. They are spawned directly by Sinister nodes, similarly to a monster spawner, and with the usual limitations including light levels. Also, if there are already four Furious zombies near the node, no more will spawn.

Wisps
Strange and wondrous creatures which spawn in magical biomes, these appear as a glowing orb flying through air and water. Each has a magical aspect, which is hinted at by their color. If killed, they will drop "ethereal essence", which bears not only the wisp's aspect, but also the precious aspect Aurum. Wisps of "orderly" aspects are neutral, not attacking unless attacked, but those of "evil" aspects may attack unprovoked. Wisps attack with lightning from considerable range, but it doesn't do much damage, and Wisps do not pursue the player very well. Note that in magical biomes, wisps also spawn underground, where they are fairly well trapped and easily killed. Wisps are common in the Nether too. The rare Obsidian Altars, generally found atop mountains, each contain a Wisp Spawner which produces only hostile Wisps.

Pech
These demi-humanoids spawn in magical biomes (including Taint, though the results aren't pretty). They come in several varieties -- most common are Foragers, but there are also Stalkers and Thaumaturges. They rush to pick up any loose items nearby, but they have a special lust for precious or magical items, and especially those with the Lucrum aspect. (It is rumored that the individual types also have other favorite aspects.)

The most common and dispensable items with Lucrum are often the golden armor from monster kills (and gold swords, once the Nether is in play). However, emeralds also serve well, advanced mages can use phials of Lucrum essentia, and any sort of mana beans will also be purchased (but are less valuable). Throwing a few such items (not mana beans) at a Pech will eventually "befriend" it, marked by green sparkles. Then the Pech can be right-clicked to open a "trading" interface. Offering another precious item and clicking the "dice" button will get you up to four items, which may be valuable or junk. (Note that items the Pech picked up will also be in the mix.) You can continue trading for several rounds, but eventually the Pech will just take the item and close the interface. You then will need to offer a few more "freebies" before the Pech will be willing to trade with you again.

"Valuable" items gained from Pech can include various magical trinkets (apprentice rings, vis amulets, and focus pouches, none very useful), silverwood or greatwood saplings, normal or enchanted golden apples, phials of Lucrum, native clusters of iron, gold, or cinnabar, and mana beans. Pech Thaumaturges can also give magical books, aspect shards or clusters, knowledge fragments, and the rare Pech's Curse wand focus.

Note that Pech are formally hostile monsters, and will commonly despawn if not properly tended. If their inventory is at least half full (five different types of items, none with Lucrum), they will not despawn, but a more reliable way to keep them around is to use a name tag on them.

Tainted monsters
Tainted lands convert ordinary mobs, including "farm animals", into horrible tainted versions. They also feature a couple of special monsters, the disturbing Tainticle and the Taint Swarms produced by Swarm Spawners. All the tainted monsters inflict the Taint status effect with their attacks, and will attack any untainted monsters or players nearby.
 * Tainted chickens, cows, pigs, and villagers are hostile and deformed, but "otherwise unremarkable".
 * Tainted creepers, when they explode, leave fibrous taint on the ground touched by the explosion.
 * Tainted sheep leave fibrous taint where they "graze".
 * Taintacles appear as a single waving tentacle. This can lash out at nearby creatures, but taintacles can also spawn smaller versions of themselves to attack at a longer range.
 * Swarm spawners appear as a glowing block, but when struck these react as a mob rather than a block. They produce taint swarms, flying clouds of glowing dots which are difficult to attack with ordinary weapons.  (However, a Sword of the Zephyr works quite well.)

Thaumic Slimes
Occasionally produced by excessive liquid ../Flux/, and when a tainted land kills any mob without a special tainted version. Like ordinary slimes, when killed they break into smaller versions of themselves (The tiny slimes can also be thrown at players.). However, thaumic slimes can also merge with each other, or grow by absorbing nearby flux.

Hellbats
Found in the Nether, these swoop at the player, filling the air with flame and eventually exploding. The blast does little damage, but delivers a startling amount of knockback. If you can find them among the flames, they are easily killed.

Eldritch Guardians
These intimidating creatures pursue those who delve too deeply into forbidden knowledge, or who interfere with certain unholy shrines. They appear as a set of ornate robes apparently floating over the ground, with no feet. However, they do not actually fly, and take normal fall damage.

In normal circumstances, they can appear as part of a ../Warp/ effect, or spawn from the altars of an Eldritch Shrine (aka Obelisk). (Inserting eldritch eyes into the altar capstones will increase their spawn rate). Either way, they bring with them a mysterious and far-reaching fog to hide within. However, eldritch guardians which appear in familiar worlds (not just the Overworld, but even the Nether and End) visibly transparent, and at a distance fade to invisibility in their fog This reflects their weakness in our dimensions, which are as hostile to them as theirs to us. Only in their home, the fabled Outer Lands, do they achieve their full strength, gaining better stats and a Runic Shield.

Eldritch Guardians have a vision range of 40 or so, but their fog extends outward to more than half that (23). If within 9 blocks of the player, they will always try to close for melee, doing damage. This is a fairly useful habit, as they are more than 2 blocks tall, and can be kept away by the same 2-block ceiling used for Endermen. From a distance, they will normally fire a dark sphere of unholy magic, which also does damage, but also inflicts 7 seconds of Weakness I.  Sometimes the dark sphere will be replaced by a "screech" -- the guardian fires a dark beam which is pretty much undodgeable, though it cannot pass through blocks. This beam does only immediately, but also inflicts Wither I for 20 seconds. The screech and perhaps the dark ball can also inflict temporary or regular warp on the victim.

They normally drop ethereal essence (Alienis, Examinis, or an "untyped" version which may be a bug). They also have a rare drop, the mysterious "eldritch eye", which can be used to activate an Eldritch Shrine.

Crimson Cult Members
The Crimson Cult can sometimes be found camping at Sinister Nodes with one or two banners. There are two variants: Crimson Knights who will chase the player and strike with melee attacks, and Crimson Clerics who shoot fireballs at the player.