One of my design goals for Solium Infernum was to keep the unit count down and to try and foster a sense of ownership and pride on the part of the player with his legions and praetors. The Attachment, Level Up and Training (Power Up) systems are intended to do this to a certain extant.
Attachments: Legions and Places of Power can receive attachments
Legion Attachments
Each legion gains attachment slots based on its level attribute. Attachment slots may be used to place Combat Cards, Praetors or Evil Artifacts. Each of these attachments provides special bonuses to the legion to which they are attached.
Legion Level 1-2: 1 attachment slot
Legion Level 3-4: 2 attachment slots
Legion Level 5-6: 3 attachment slots
Legion Level 7+: 4 attachment slots
Note: The details of the bonuses provided by Combat Cards are only viewable to the owner of the legion. The details of any Praetors or Evil Artifacts attached to legion are viewable by opponents.
Level Ups: Legions and Praetors can level up
Legion Level Up
When a legion levels up after a victory in battle, the owner is sent a message that appears in the owner’s turn log. The owner must click the response button for that message and select the upgrade bonus that they desire. Upgrade bonuses are randomly determined and consist of increases in combat attributes, loyalty, move points or new special abilities. The owner must select one and only one of the three choices offered. The level up bonus is applied at the commencement of the processing of the following turn.
Praetor Level Up
When a praetor levels up after a victory in single combat, the owner is sent a message that appears in the owner’s turn log. The owner must click the response button for that message and select the upgrade bonus that they desire. Upgrade bonuses are randomly determined and consist of increases in single combat attributes (skulls, shields, orbs or hit points). The owner must select one and only one of the three choices offered. The level up bonus is applied at the commencement of the processing of the following turn.
Training: Legions and Praetors can be trained via Manuscripts
Legion Training
Some rare manuscripts that can be found through tribute or bought in the Infernal Bazaar allow legions to upgrade their attributes. This is done by issuing an order by clicking on the legion icon or through the “Ministerium” panel. The manuscript is used like a normal tribute card to pay for the training and the training upgrade is applied at the commencement of the processing of the following turn
Praetor Training
Some rare manuscripts that can be found through tribute or bought in the Infernal Bazaar allow praetors to upgrade their attributes and more importantly add special combat moves. This is done by issuing an order through the “Ministerium” panel. The manuscript is used like a normal tribute card to pay for the training and the training upgrade is applied at the commencement of the processing of the following turn

These are single card manuscripts that can be used to train Praetors. Most add a special combat move to the Praetor for use in single combat. Some like the Manual of Attack increase the Praetor’s attributes.
Manuscripts can also come in two parts that must be collected and used together for training:

Notice that the red letter and number in the bottom corner of the manuscript designate whether the manuscripts trains a “L” legion or a “P” praetor and which section of the manuscript it is.
Although there are a few 3 part manuscripts for training Praetors with rare and powerful combat moves the majority are used for performing Unholy Rites. The Unholy Rites can be performed only if you have collected all three parts of the manuscript and they result in the gain for you of two points in a specific attribute associated with the rite (but the attribute can never be raised above 6). The Rite of Blood increases your “Wickedness” attribute and therefore also your “Destruction” power by 2 points. The rite also often grants you a small windfall of tribute cards.

The most difficult of all the manuscripts to collect and assemble is the 4 piece Infernal Machine

The Infernal Machines can be constructed only if you have collected all four parts of the manuscript and they result in your opponents (but not your Blood Lord/Vassal if any exist) losing 2 points in a specific attribute associated with the machine. The Machine of Gluttony decreases all your opponents’ “Charisma” attribute and therefore also their “Diabolism” power by 2 points. The successful collection and assembly of all four pieces by an Archfiend is a rare, momentous and agonizing event for every denizen of hell.