The Spartakusbund doesn't remain a faction of USPD for that long. Instead, they form a communist party in 1918 before (or shortly after) the november revolution begins. A unified program is created that openly advocates for a council republic and states that socialism can only be created through the continuation of the revolution (i.e. expropriation of the burgeoisie, establishment of a proletarian democracy, abolishion of the military and the status of civil servants in it's burgeois form, etc.). Organized in one party, the revolutionary socialist movement has a lot more possibilities when it comes to agitation and propaganda. The supporters of a socialist council republic , instead of beeing splintered over many different factions (in OTL there were supporters of a council republic within the SPD, USPD and Revolutionäre Obleute, but they were disorganized), actually work together. In the end they manage to spread class consciousness way more effective than in OTL and gain a majority in the Reichsrätekongress (it was a lot more of a close call than the final result makes it seem. In the night before the voting on this issue, Ebert send his men into allmost all factories of Berlin, spread lies about the revolutionary socialists, made empty promises and even outright bribed deputies to vote for him. At the same time, people like Liebknecht had very limited possibilities to spread their ideas, and they were relatively unknown untill after the november revolution had begun). The forces of reaction gather and there is a right wing coup (similiar to the OTL Kapp Putsch but more powerfull and with the support of the SPD leadership). Yet a general strike defeats it. Freikorps continue to tyrannize the people for another couple of months, but they're disorganized after the failed coup and the expropriation of their backers and so are eventually crushed by the revolutionary army. The Reichsrätekongress gathers once again, declearing their loyalty to the Free Socialist Republic or Germany that Liebknecht had decleared. Karl Liebknecht is furthermore elected as the chairman of the Zentralrat der Sozialistischen Republik Deutschlands. A new Germany has been born. One of the first actions of the new government is to send material support and volunteers to the Bolsheviks in Russia.
The funny thing about the november revolution is that the absolute majority of the people in general (and the workers specifically) wanted socialism. And the SPD, USPD, Spartakusbund and Revolutionäre Obleute all promised to construct socialism in Germany. Yet Ebert and his men never intended to hold their promise. They decleared that socialism could be reached through the establishment of a burgeois-democracy, and many people believed him. In the end the german workers got capitalism with a few more concessions than in, say, France or Britain.