Both of these are martial classes, which helps a fair bit with synergy on a base level. However, they are both two-ability classes, with the bloodblade favoring Constitution and Intelligence and the herald favoring Strength and Charisma. The use of the bloodblade’s Power of the Mind feature can mitigate this somewhat, by tying weapon attack and damage to Intelligence rather than Strength or Dexterity. Flavor-wise, this is a really fun combination. There’s a certain Gothic vibe to be had here with a character modifying their blood to help with their ideological crusade, changing their very essence in order to better fulfill their goals and perform their mission. Pretty much any mix of these two classes will play similarly; the character will function best when they can engage in close combat. The biggest differences you will see is the mix of specific tools at the character’s disposal. More herald levels means more magic and probably more exertion, more bloodblade levels means a larger pool of hit points and greater access to the various buffs and debuffs in the bloodblade’s arsenal. [[Synergy Feats for Multiclassers#Bloodblade w/ Herald|Bloodblade-Herald chains and ideas]]