As with every other guide, I will open by shilling the Rusted Pack File Manager[github.com], hereafter referred to as RPFM. It is BY FAR the best modding tool in your arsenal, vastly superior to the formerly popular PFM which hasn't been seriously updated in a long time whereas RPFM is actively updated and has many many features PFM does not. I will not be explaining the basics of RPFM in this guide though, I've done it too many times in previous guides and there are other excellent resources out there like Vandy's modding basics guide to teach it.

Ancillaries are a catchall term for the various item types that you can get - particularly mounts, items, banners, and followers. Some 'items' in vanilla are actually just skills, like Teclis's Potion of Charoi, but in general if you can equip it, it's an ancillary.

As usual, your key needs to be unique and copied to notepad or w/e since you'll be referencing it a lot. Most of the fields here are deprecated from older games and easily solved by copy-pasting from a vaguely similar item, but some important columns...

• Type is the icon for the type of ancillary you're making; wh_main_anc_arcane_item, wh_main_anc_weapon, wh_main_anc_talisman, wh_main_anc_enchanted_item, wh_main_anc_armour are the item types, but they can also be various kinds of followers, mounts, etc. Plenty of vanilla examples!
  
• Transferrable means tradable - leave empty if it's a unique item/mount/whatever unless you WANT your lord to be able to give it away!
  
• Category is the actual database 'wtf is this item' column. It should obviously roughly match Type. Note that followers and banners are actually General type, but the rest are pretty self explanatory and easy to find.
  
• Provided Bodyguard Unit is used for mounts and links to the main_unit of the mount. Remember that the mounted unit replaces the base one! You can also add bodyguards here, but it works realllly weird and I'll go into more detail later.
  
• Uniqueness Score is 200 for legendary lord items as a general rule of thumb, and decreasing rarity from there.

This just links the ancillary to its description.

Basically, what subculture does this ancillary go to? Just copy a vanilla one. I don't think this table is NECESSARY per se, but if you want to add generic items to Chaos to the game, this ensures they don't show up on Empire lords.

If you only want a specific character or character type to be able to equip this, say, ONLY King Louen can equip the new mount you're making for him, this is your table. Primarily used for unique weapons and items, like Karl Franz's Ghal Maraz.

This is where you attach effects like melee attack or leadership to army. Scope and Value work the same way as they do in every other X to effects table. If you're not familiar with effects, read my effects guide.

Assuming you're just using an existing mount, this is where you link the new mount unit to the existing variant stuff.

There are two kinds of mounts; the ones that exist ingame as mounts and are easy to set up, and something like trying to make a Demigryph a ridable campaign mount for Empire Generals. The former is easy; the latter is doable but enough of a pain in the butt that I won't cover it here.

If you just want to, say, add a Star Dragon as a mount to Malekith, though, that is fairly easy.

You will need to make a custom land_unit and main_unit for this mount - let's call it def_malekith_star_dragon for now, since this is the unit for Malekith riding his pimpin' new dragon. Fill it out similar to other dragon mounted characters, like for example Malekith's own Seraphon mount, wh2_main_def_cha_malekith_3 for reference. Note that you may have to play around with animation tables a bit if you're doing weird stuff like Eltharion on a Griffon.

Make sure that the Mount column references an existing mount - this will save you approximately 18 years of work. Check the mounts table if needed to find the key, in our case, wh2_main_hef_mnt_star_dragon.

Remember that Melee Attack, Melee Defense, Hit Points, Attribute Group, Damage Mod Physical, etc. all work the same as they usually do and will REPLACE the lord's base values! In other words, it doesn't matter if Malekith normally has 5 or 500 Melee Attack, only this unit's MA matters while he's mounted. Similarly, most lords on large mounts gain the Siege Attacker attribute and thus will need a custom attribute group assigned to them here.

Officers is not always needed, but some kinds of mounts (particularly dragons) do need it. For our specific example, we already have a Malekith officer to work with, you may have to make your own if you're doing something weirder. Same with Articulated Record and War Machine if you want to add, say, a Flame Cannon mount to a Dwarven Master Engineer.

If you are actually adding bodyguards, you will 100% have to mess around with the officers table. Remember that you can't give unique stats to each unit in the group - so you will probably need to significantly nerf damage etc. to account for the number of units in the special bodyguard unit. 3x200 is still better than 1x500 even if it looks weird.

Once you finish with the Land/Main units and link them properly, make sure that the ancillaries table links to the new Main Unit as a bodyguard unit.

The campaign_mount_animation_set_overrides table is used to actually change your appearance on the campaign map while mounted.

As usual with ancillaries, you'll need to make a custom skill and link it to an ancillary normally - DON'T be the douchecanoe that reuses an existing skill, say, the normal rat warlord bonebreaker mount. That skill is linked to the generic bonebreaker and will cripple your lord's uniqueness and stats when mounted. Be SURE that you are giving the correct ancillary which in turn has the correct main unit as a bodyguard!

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As one final note; if you are doing something like the aforementioned 'demigryphs for empire generals' thing, you will need to fill out a number of mount, battle entity, variants, etc. tables. Just try to follow key links.

After you do all the stuff in the above section, you may want to test your mount out in custom battles. That's cool! You just need a few more tables for that.

Similar to how we filled out land_units and main_units, you will need separate entries in units_custom_battle_permissions for each mount the lord has. They can reuse the same portrait.

You will also need to fill out units_custom_battle_mounts to properly attach the mounts to the base character.

Finally, you'll also need to fill out custom_battle_mounts.loc in order for said mounts to have names and descriptions.

Ancillaries have two loc fields needed - ancillaries_onscreen_name_ is the prefix for its name, while ancillaries_colour_text_ is the prefix for description, as always they go in different rows.