Adding another slot makes creating Tier 3 units a lot easier. This guide on How To Get A 4th Ingredient Slot For Necromancy In Spellforce Conquest Of Eo explains the process you need to complete in order to increase the number of materials you can use in Necromancy from three to four.
It is worth noting that all of the Necromancy recipes, including Tier 3 units, can be achieved using just the base three ingredients slot that you start the game with. Obviously, however, this does mean that you will need higher quality ingredients in order to reach some of the more challenging undead you can create, but you do not need the 4th ingredient if you have already progressed far enough into the game to get the more valuable resources.
How To Get A 4th Ingredient Slot For Necromancy In Spellforce Conquest Of Eo
If you want to unlock the 4th ingredient slot for Necromancy, open up your Grimoire. Once in the Grimoire use the tabs at the bottom of the screen to navigate to the Necromancy page, the icon looks like a spooky hand coming out of the floor. There are several challenges here and each challenge has a reward. The first three challenges are for spells. You get Summon Necromancer Minions, Soul Cage, and Soul Catcher, after completing those first three challenges.
The final challenge, however, unlocks a special room called the Soul Well. Once you have unlocked this room and you add it to your Tower, you will get new rewards including weekly souls, an additional stack slot for your armies, and also the ability to use a 4th ingredient when summoning undead creatures through Necromancy.