Holding Carnage alone doesn't do anything but when swung it causes the whole server to crash. Other weapons and items seem to function fine. The weapon also works fine in singleplayer.
This is from the crash report

Description: Exception in server tick loop

java.lang.NoSuchMethodError: net.minecraft.class_1657.method_18381(Lnet/minecraft/class_4050;)F
at draylar.crimsonmoon.item.CarnageItem.attack(CarnageItem.java:39)
at draylar.crimsonmoon.network.ServerNetworking.lambda$null$0(ServerNetworking.java:23)
at net.minecraft.class_3738.run(class_3738.java:18)
at net.minecraft.class_1255.method_18859(class_1255.java:144)
at net.minecraft.class_4093.method_18859(class_4093.java:23)
at net.minecraft.server.MinecraftServer.method_24306(MinecraftServer.java:761)
at net.minecraft.server.MinecraftServer.method_18859(MinecraftServer.java:155)
at net.minecraft.class_1255.method_16075(class_1255.java:118)
at net.minecraft.server.MinecraftServer.method_20415(MinecraftServer.java:743)
at net.minecraft.server.MinecraftServer.method_16075(MinecraftServer.java:737)
at net.minecraft.class_1255.method_18857(class_1255.java:127)
at net.minecraft.server.MinecraftServer.method_16208(MinecraftServer.java:722)
at net.minecraft.server.MinecraftServer.method_29741(MinecraftServer.java:674)
at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:257)
at java.lang.Thread.run(Thread.java:748)

Im not sure if this is caused by another mod but I'll give a list here
Fabric Mods:
adorn: Adorn 1.12.2+1.16.4
autoconfig1u: Auto Config v1 Updated 3.3.1
bettercaves: YUNG's Better Caves 1.16.2-1.0.3
betterend: Better End 0.9.1-pre
betternether: Better Nether 5.0.4
byg: BYG 1.1.5
cardinal-components: Cardinal Components API 2.7.9
cardinal-components-base: Cardinal Components API (base) 2.7.10
cardinal-components-block: Cardinal Components API (blocks) 2.7.9
cardinal-components-chunk: Cardinal Components API (chunks) 2.7.9
cardinal-components-entity: Cardinal Components API (entities) 2.7.9
cardinal-components-item: Cardinal Components API (items) 2.7.9
cardinal-components-level: Cardinal Components API (world saves) 2.7.9
cardinal-components-scoreboard: Cardinal Components API (scoreboard) 2.7.9
cardinal-components-util: Cardinal Components API (utilities) 2.7.9
cardinal-components-world: Cardinal Components API (worlds) 2.7.10
cavebiomes: Cave Biomes 0.5.1
charm: Charm 2.2.2
cloth-api: Cloth API 1.5.47
cloth-armor-api-v1: Cloth Armor API v1 1.5.47
cloth-basic-math: Cloth Basic Math 0.5.1
cloth-client-events-v0: Cloth Client Events v0 1.5.47
cloth-common-events-v1: Cloth Common Events v1 1.5.47
cloth-config2: Cloth Config v4 4.10.13
cloth-datagen-api-v1: Cloth Datagen v1 1.5.47
cloth-durability-bar-api-v1: Cloth Durability Bar API v1 1.5.47
cloth-dynamic-registry-api-v1: Cloth Dynamic Registry API v1 1.5.47
cloth-utils-v1: Cloth Utils v1 1.5.47
com_electronwill_night-config_core: core 3.6.2
com_electronwill_night-config_toml: toml 3.6.2
com_moandjiezana_toml_toml4j: toml4j 0.7.2
craftycuisine: Crafty Cuisine b1.1
crimsonmoon: Crimson Moon 2.0.1-beta-1.16.5
eldritch_mobs: Eldritch Mobs 1.6.8
extraorigins: Extra Origins 1.16.4-6
fabric: Fabric API 0.30.3+1.16
fabric-api-base: Fabric API Base 0.2.1+9354966b7d
fabric-biome-api-v1: Fabric Biome API (v1) 3.1.1+ca58154a7d
fabric-command-api-v1: Fabric Command API (v1) 1.0.10+ca58154a7d
fabric-commands-v0: Fabric Commands (v0) 0.2.2+ca58154a7d
fabric-containers-v0: Fabric Containers (v0) 0.1.11+9354966b7d
fabric-content-registries-v0: Fabric Content Registries (v0) 0.2.1+ca58154a7d
fabric-crash-report-info-v1: Fabric Crash Report Info (v1) 0.1.3+ca58154a7d
fabric-dimensions-v1: fabric-dimensions-v1 2.0.6+9354966b7d
fabric-entity-events-v1: Fabric Entity Events (v1) 1.0.3+ca58154a7d
fabric-events-interaction-v0: Fabric Events Interaction (v0) 0.4.2+ca58154a7d
fabric-events-lifecycle-v0: Fabric Events Lifecycle (v0) 0.2.1+ca58154a7d
fabric-game-rule-api-v1: Fabric Game Rule API (v1) 1.0.6+ca58154a7d
fabric-item-api-v1: Fabric Item API (v1) 1.2.1+ca58154a7d
fabric-item-groups-v0: Fabric Item Groups (v0) 0.2.3+ca58154a7d
fabric-language-kotlin: Fabric Language Kotlin 1.4.21+build.1
fabric-lifecycle-events-v1: Fabric Lifecycle Events (v1) 1.2.1+ca58154a7d
fabric-loot-tables-v1: Fabric Loot Tables (v1) 1.0.2+ca58154a7d
fabric-mining-levels-v0: Fabric Mining Levels (v0) 0.1.3+ca58154a7d
fabric-networking-api-v1: Fabric Networking API (v1) 1.0.1+ca58154a7d
fabric-networking-blockentity-v0: Fabric Networking Block Entity (v0) 0.2.8+ca58154a7d
fabric-networking-v0: Fabric Networking (v0) 0.3.2+ca58154a7d
fabric-object-builder-api-v1: Fabric Object Builder API (v1) 1.9.4+9354966b7d
fabric-object-builders-v0: Fabric Object Builders (v0) 0.7.2+ca58154a7d
fabric-particles-v1: Fabric Particles (v1) 0.2.4+ca58154a7d
fabric-registry-sync-v0: Fabric Registry Sync (v0) 0.7.4+ca58154a7d
fabric-rendering-data-attachment-v1: Fabric Rendering Data Attachment (v1) 0.1.5+ca58154a7d
fabric-resource-loader-v0: Fabric Resource Loader (v0) 0.4.2+ca58154a7d
fabric-screen-handler-api-v1: Fabric Screen Handler API (v1) 1.1.5+9354966b7d
fabric-structure-api-v1: Fabric Structure API (v1) 1.1.4+ca58154a7d
fabric-tag-extensions-v0: Fabric Tag Extensions (v0) 1.1.1+ca58154a7d
fabric-tool-attribute-api-v1: Fabric Tool Attribute API (v1) 1.2.6+ca58154a7d
fabricloader: Fabric Loader 0.11.2
fallflyinglib: FallFlyingLib 1.1.0
jankson: Jankson 3.0.1+j1.2.0
java: OpenJDK 64-Bit Server VM 8
libcd: LibCapableData 3.0.3+1.16.3
libgui: LibGui 3.1.0+1.16.3
libstructure: Libstructure 1.3
lightstones: Lightstones 1.3.0
lithium: Lithium 0.6.0
lithium-api: Lithium API 0.6.0
medievalweapons: Medieval Weapons 1.0.7
minecraft: Minecraft 1.16.4
mostructures: Mo' Structures 1.0.1+mc.1.16.4
org_jetbrains_annotations: annotations 20.0.0
org_jetbrains_kotlin_kotlin-reflect: kotlin-reflect 1.4.21
org_jetbrains_kotlin_kotlin-stdlib: kotlin-stdlib 1.4.21
org_jetbrains_kotlin_kotlin-stdlib-jdk7: kotlin-stdlib-jdk7 1.4.21
org_jetbrains_kotlin_kotlin-stdlib-jdk8: kotlin-stdlib-jdk8 1.4.21
org_jetbrains_kotlinx_kotlinx-coroutines-core-jvm: kotlinx-coroutines-core-jvm 1.4.2
org_jetbrains_kotlinx_kotlinx-coroutines-jdk8: kotlinx-coroutines-jdk8 1.4.2
origins: Origins 0.4.7
origins-classes: Origins: Classes 1.1.0
pehkui: Pehkui 1.7.1+21w03a
phosphor: Phosphor 0.7.1+build.11
polymorph: Polymorph 0.0.6-1.16.4
reach-entity-attributes: Reach Entity Attribute 1.0.1
seasons: Fabric Seasons 1.0.3-BETA
sit: Sit 1.16.2-9
tk_valoeghese_zoesteriaconfig: ZoesteriaConfig 1.3.2
zoob: Monster Of The Ocean Depths 3.0.0

Let me know if you need more info!

Thank you for the report, fixed in the next release!