The Soulvizier mod aims at providing a more varied and difficult TQ:IT experience. Instead of focusing on a single aspect, the mod expands on just about everything in the game: New items, new monsters, new skills, new areas and more. Built upon Munderbunny's highly popular Underlord mod, it includes all his mastery reworks and new content.

Features:

180+ New hero monsters.
180+ Items.
New item types (Souls, Mercenary Scrolls, Orbs etc.)
500+ Souls.
Elite monsters.
All bosses rebalanced.
Greatly increased difficulty.
New Merchant class "Alchemist".
180+ Item skills.
Map and AI enhancements.
Build upon Munderbunny's Underlord mod.
Includes Kirii's DiabloUI.
Gameplay Changes

Base move/attack/cast speed increased.
Max level raised to 1000.
Epic and Legendary items are enchantable.
Changed XP formula to make late game leveling faster.
Souls
Souls are ring-slot items which inherit attributes and even sometimes abilites from their former owners. Each monsters has its own soul which only it can drop.
Example:

If you've played the Nez mod for D2 you will be familiar with the concept of souls. Souls from bosses and hero monsters are especially powerful and their potential should not be overlooked.
Souls are quite rare and are used in various useful and powerful recipes and should therefore always be picked up, even if their bonus stats seem weak.

Mercenary Contracts
Mercenary Contracts are rare artifact-slot items which offer powerful summonable pets.
Each mercenary is specialized at a certain role and their power easily rivals that of other artifacts.

Hero Monsters
There's over a hundred new hero monsters and existing heroes have all been buffed both in terms of power and loot drops. All heroes have their own visuals and combinations of abilities, some of which can be obtained from their souls. A few very powerful uber heroes can also rarely be encountered.

Orbs
Orbs are rare one-handed items, some act as shields others as weapons, often providing powerful elemental or spiritual bonuses at the cost of reduced physical protection.

Forge Formulas
Forge Formulas are recipes which are used at Enchanters in the same way as Arcane Formulas, however these formulas aren't used to forge Artifacts, but can instead be used to create wide variety of powerful and specalized equipment.