Okay, I will explain how to play as the BE. I found it out by searching "how to play as Romano British in BI", for Rome Total War. It worked for me. It's not a big deal but as I couldn't find out how easily I decided to write this guide. Here are the links that guided me when doing this:
http://www.twcenter.net/forums/showthread.php?90329-What-can-i-do-to-play-Romano-British-in-BI
https://forums.totalwar.org/vb/showthread.php/51370-How-To-Change-the-ownership-of-provinces?p=676346#post676346
All right, so, before doing it you should know which are the main differences between the High Elves (HE) and the Blood Elves (BE). They just have 3 different units, plus the color changes from blue to red. Those units (which you can check on the custom battle screen) are:
-Knights of Quelthalas-->Mounted Blood Knights
-QuelDorei Dawnrunner-->Elven Bloodshade
-Foot Knights-->Blood Knights
Check if that's enough for you just to want a change. In case you do here's how to do it.
First of all, what we will be creating is a sub-mod so we don't change the main one. We need to install the mod again but in a different DLC folder. Mine was on “teutonic”, so now I Will install (or just copy the mod files) in “crusades” (empty that folder, then paste the mod files inside). Now go to C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data\world\maps\campaign\imperial_campaign, where the main campaign files are placed.
NOTE: this is where my game is. It's the Steam version installed over the crusades mod. Yours may be somewhere else. From now on I will be refering to this adress because of that.
For the next steps keep in mind that the HE are called "quelthalas" in the files, and very important, the BE are called "venice". Whenever you see venice in the files, those are the Blood Elves.
1-Make the faction playable. This is an old trick on the Total War series: go to C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data\world\maps\campaign\imperial_campaign and open the file called "descr_strat". Now CUT venice and paste it on the playable list. In case you try in the future to make the Forsaken playable you must do this too with them (or do it right now).
2-Go to C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data and open “descr_sm_factions” and look for venice. It will look like this:
faction venice, spawned_on_event
Remove ", spawned on event". Remember to remove the comma. Now the BE won't trigger on any campaign. This is why we are doing this as a sub-mod, because we could do it just as a custom campaign but then the BE wouldn’t trigger in the original campaign. We will also be removing the "dead_until_resurrected" line from
faction venice, fortified genghis
ai_label default
dead_until_resurrected
denari 10000
denari_kings_purse 3000
in descr_strat which is in C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data\world\maps\campaign\imperial_campaign. It should look like this:
faction venice, fortified genghis
ai_label default
denari 10000
denari_kings_purse 3000
3-Now we will give them provinces, armies, a family tree and the agents. Open descr_strat in C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data\world\maps\campaign\imperial_campaign where those things are. Now we will just CUT the lines from quelthalas and paste them under venice. First, look for quelthalas and cut from the world "settlement" to "end". We are copying now the settlements, the characters, the agents and the family tree (last line, where the "end" is) and take from quelthalas all those things. Now paste it right under
faction venice, fortified genghis
ai_label default
dead_until_resurrected (shouldn't be here by now)
denari 10000
denari_kings_purse 3000
(here)
Take a good look at what you just pasted because you can do a lot if you modify this for other factions.
NOTE: the names of the family (and maybe the agents) must match with the one available for that faction. In this case they are the same so don't worry. Just for the ones who want to play as Forsaken, the names are different. You can see each faction's lists in the file "descr_names" in C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data.
NOTE: you may want to change the gold you start with.
4-We must change the names of the units to match the ones they can produce. I'm not sure how important is this, but it didn't crash for me and I did it. So go to what you just pasted. There we will replace the names of some units to their BE version (or whatever other unit you want). The units are the mentioned above. In descr_strat change your armies’ names like this:
-Knights of Quelthalas-->Mounted Blood Knights
-QuelDorei Dawnrunner-->Elven Bloodshade
-Foot Knights-->Blood Knights
In some cases the names of the general's bodyguard can change. Not in this case, but be aware of it (i.e. a general has Foot Knights as bodyguard, you must change it to Blood Knights).
5-Delete the file map.rwm placed on C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data\world\maps\base - a new one will be created next time you play.
6-Profit.
ADITIONAL NOTE: some factions can recuit certain units depending on where they are. I mean, like recuiting mages as Lordaeron in Dalaran and nowhere else. It's not the case for both the HE and the BE but I will tell you how this works: to produce the unit the province must have certain hidden resource. It's a requirement. The buildings are on "export_descr_buildings" in C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data. There you can see the buildings and "hidden resource" for some special units. I haven't touched this but I guess you could remove it to recuit them. Another option is to add that resource to the provinces we want. For that, we go C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods\crusades\data\world\maps\base and open "descr_regions". Some of them have those hidden resources (but the word "hidden" won't appear). I.e. Grim Batol has some hidden resources. This is what it looks like:
Grim_Batol_Province
Grim_Batol
goblin
Darkhorde_Rebels
0 76 106
troll, deadtroll, darktroll, deadorc
5
5
religions { light 0 sun 0 moongodess 0 titan 0 shaman 100 undeath 0 legion 0 naga 0 troll 0 heretic 0 }
The hidden resources here are "troll, deadtroll, darktroll, deadorc" and maybe goblin. Adding the ones you want wherever you want would allow you to recuit certain units in different provinces.
It’s the first time I write a guide (as you can see) and I guess there are many mistakes. Just let me know if you find any. I will try to upload the files somewhere as son as I know where and how.