Cards work similar to armor and ornaments. They add white and blue stats they carry.
In PWI you can devour lower card by higher one to increase XP of higher card. Same as in
those Card Battle games on iPad and Android. I was told that here each card is already maxed,
no need to use Devour to increase its XP. That thing was done for balance.
In order to work, each card has to be in corresponding slot.
For example, Destroyer card works only in Destroyer slot, Battle card only in Battle slot and so on...
You can put cards to "wrong" slots, but they add nothing while there.
Cards have levels: C, B, A, S, S+. Cards level C are the lowest, Cards level S+ are the highest.
As far as I can see best are level S, because S+ were added just to allow getting highest titles for
collecting numbers of cards, but from S+ their white stats were removed.
White stats on cards are same for "same slot card" of same level. Blue stats vary. Anyway, it is
better to have A card of "bad" blue stats with better white stats, than B card of "good" blue stats
on lower white stats. Of course the best is to have higher card with "good" blue stats.
For example, you can add card with Str to Wizard, and move your own Str points to Mag.
But if you replace card later, you will have to reset points again. I'm too lazy to do it, so I
use Mag card on mage class, Dex card on LA class... Unless put Atk Level, Def Level, Crit, ...
Slots also get some points when you use Nuema. You can get nuemas from nuema boxes, or from
bosses in low level caves: FB29 (Qingzi), FB39 (Hercule Trioc), FB51 (Wyvern), FB59 (...),
FB69 (Cenequus Polearm), FB79 (Linus the Woeful). Maybe others there too, I don't know, sorry.
Some cards require Leadership 1, some require 2, some require 3. Your leadership is shown at the
very top of the War Avatar window. Starts with 10 and you can add 8 more, 1 by 1, in Primal World
at War Avatar Master. From God's Giving buy War Avatar Catalyst, then at War Avatar Master take
quests as they appear: for 10 Catalyst, for 20, 30, ..., 80. You will need 360 Catalyst in total, they
all together cost about 18-19 million coins. (It was 37 mill 2-3 weeks ago, but Agatio helped.)
All the beast and good luck in getting good cards. Browse Crimson Officer Ling in Cromagnon Village
to see what packs he can offer. You have S packs for 4 Flowsilver Coin each. Unlike other S pack this
one gives S cards only. Other one gives much more B, few A and scarce S.
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I hope this description was short enough.