List of most powerful and notable competing civilizations of the interrelativistic war, their basic descriptions, and posed social threat:
(Note that a civilization must develop or adopt interrelativistic travel to become contenders on such a scale.)
Kardashev scale key (each rank means to harness all the energy of its reference. The rank means to harness the entirety of the implied, ie. having fusion power from an artificial source like a reactor does not mean you are Kardashev 2):
Kardashev 0: [N/A]
Kardashev 1: [PLANETS]
Kardashev 2: [STARS]
Kardashev 3: [GALAXIES]
Kardashev 4: [UNIVERSE]
Kardashev 5: [UNIVERSES]
Kardashev 6/Ω: [MULTIVERSE/TRANSCENDS]

CIS (Confederacy of Independent Systems)
Imperialistic hierarchy multirace biomechanical partial hivemind
Kardashev 2
Ethnocentric converters (culture based, not xenocidal)

The United Galaxies
Imperialist Hierarchy, biomechanical technology
Kardashev 3
Subjugation and xenocide
Known members:
Amphicephali
Asteromorphs
Terrestrials
New Machines
Subject Descendants

FOIL (Federation of Intelligent Life) (Possible affiliation with Combine)
Multirace Imperialist hierarchy. Biomechanical technology
Kardashev 3, possible 4
Destructive exploitation
Known members and observed social structure (ranked in hierarchal order, similar roles means same indoctrinated level.):
-Kaleesh, independent supreme command, military leadership, warlord civilization
-Heptapods, non-combatant, pan-dimensional advisory
-Cheela, non-combatant, from the neutron star Dragon's Egg, hyper advanced engineers
-Cultivated bioengineered service laborers, military, and domestic
-Artificial new species from enlisted labor classes by cybernetic and chemotherapeutic enhancement as units for warfare
-version of Sangheili, non-combatant, management and projected upper-class
-version of Humanity, non-combatant, labor species and partial management
-minority version of Unggoy, non-combatant, labor species and partial management
-minority version of Kig-Yar, non-combatant, labor species and partial management

The Flood
Seemingly instinctual fungal based parasites. bio-assimilators of organic material. Biomechanical technology.
Kardashev 2
Threaten biological and cultural diversity

Gracatrons
Cybernetic reptilian hivemind. Borg-like assimilators (possible descendants from a successful version of the Borg)
Kardashev 2
Xenocidal against all other civilizations.

Necromorphs
Seemingly instinctual bacterial based parasites. Bio-recombinators of inactive gene-based organic material. Use focal units to catalyze spread. Biomechanical technology.
Kardashev 2
Threaten biological and cultural diversity

Primes
Instinctual imperialist biological hierarchal hivemind, biomechanical technology
Kardashev 2
Driven by instinct to establish total domination to ensure survival
Two classes:
Immotiles: naturally immobile thinking units
Motiles: naturally mobile labor units

Thordonians
Imperialist biologically derived digital consciousness. Biomechanical technology
Kardashev 2
Extreme self-righteousness. Cultural destruction and xenocide

(I am open to adding your civilizations to this list, although I hope they are considerably powerful or adaptive to compete properly.)