Split Faction refers to a deck, of which half of the runes are of one faction, and half the runes of another faction. This type of deck, and Full Faction decks, are the only deck structures currently allowed in Ranked Play, and are considered to be "regulation" decks. The Training Grounds, Daily Missions, and Campaigns all allow the use of "non-regulation" decks.
A Split Faction deck will recieve a reduced version, of each faction used in the Split Faction deck.
In addition, a split faction deck will only allow you to use one single split-hero rune from each Rune-Group. For example if you are playing a IS/ST deck, you couldn't use both an IS/ST and an IS/UD split-hero runes, as the limit for IS split heroe runes would be one.
The following are the Split Faction bonuses available from each of the 8 factions.
SL - Champions you control take 10% less damage (after DEF), with a minimum reduction of 1, from attacks.
FS - You recover 3 nora at the start of each turn.
FW - Champion runes you own have their cooldowns reduced by 30%
IS - Champions you control take 3 less damage from spells.
KF - Champions you control have +1 maximum AP.
ST - Champions you control have +5 maximum HP.
SP - Whenever you deploy a Champion, you gain 4% of its Nora cost. When that Champion is destroyed, you gain 2% of its Nora cost.
UD - Champions you control with a maximum range of 3 or less receive a +2 DMG buff. Champions you control with a maximum range above 3 receive a +1 DMG buff.