Blood Phage adds a nice flexible econ attacker to red. Utilizing both red produced by an Animus in a low econ game, especially double Animus with Shadowfang, can sometimes be difficult without cheap red sinks like Husk or Perforator. Now with Blood Phage you can grab an attacker that can give you an easy way to convert spare red to gold making it easier to buy more red next turn. This can also combine well with a Flame Animus providing 2 attack and 2 gold on turns when you don’t need that red. Let’s look at some other units that are likely to factor into using the Phage.
Units usually needing double animus (3-4 red cost)
Corpus helps build some good granular defense for double animus builds, combined with Phage could use the spare red to build a 1 turn combo attack and econ unit for cheaper than 2 tarsiers. Especially useful as a way to spend energy later in the game when a drone is less useful than an attacker.
Tatsu won’t leave you with leftover red on the turn you buy it, but for when you run out of supply or can’t afford to buy one on a turn, having Phages to do something useful with the excess red should come in handy, especially in sets without any red units cheaper than tarsier.
Excess red with Lucina will often be spent to power the Perforators, but for the times when you can’t spend all 4, it will be helpful to have the Phages since you’ll be sacrificing drones to build your army.
Blood phage will also be competing with some other cheap red sinks so keep an eye for when the red might be better spent on one of these.
Other uses for spare red
Frostbite can be a decent way to spend excess red to build up your threat power, but the build time can put a damper on this being as much of a sink. It also doesn’t generate anything while it sits idle except for getting the opponent to over defend.
Much like a Pixie is for blue, Nitrocybe is a good way to convert excess red into a flexible burst of attack. On the plus side, if you have a Phage and 2 red but 0 gold, you can use its conversion to essentially buy a nitrocybe for just the 2 red.