Both are S-tier Arcane weapons with devastating AoE weapon skills. Mohgwyn's Sacred Spear uses fire + bleed AoE; Rivers of Blood uses multi-hit bleed combos. Both melt everything, but differently.
Mohgwyn's Sacred Spear
AoE bleed against groups and large bosses
Strengths
- Bloodboon Ritual has the largest AoE of any bleed weapon
- Fire + bleed combination damage
- Great spear range for safe poking
- Excellent against large boss hitboxes
- Strong in invasions against groups
Weaknesses
- Requires defeating Mohg
- Higher Strength requirement
- Ritual has a windup time
- More Strength/Arcane split stat needs
VS
Rivers of Blood
Fast single-target bleed and PvP
Strengths
- Corpse Piler is faster than Bloodboon Ritual
- Better 1v1 pressure in PvP
- Katana moveset is faster for regular combat
- Lower stat requirements
- More consistent bleed application
Weaknesses
- Must be obtained before killing Fire Giant
- Nerfed in 1.06
- Less effective against groups
- Lower AoE range than Bloodboon Ritual
Verdict
Mohgwyn's Sacred Spear wins for AoE situations: groups of enemies, large bosses, and invasions against ganks. Rivers of Blood wins for single-target: 1v1 PvP, smaller bosses, and general exploration. Both are S-tier; pick based on your preferred engagement style.