Fighting Alongside Death is a quest given to Byleth by Jeritza in Fire Emblem: Three Houses during the War Phase of the Crimson Flower Route. Unlike other quests the only participants are Byleth and Jeritza.
"Secret orders from the emperor. Do not keep me waiting."
"Go into battle and subdue the enemy. This is a covert mission, so you'll be pairing with only Jeritza."
Quest-Giver Dialogue 
"Secret orders from the emperor. You and I are to launch a surprise attack on the enemy base in the eastern part of Fódlan. Come."
Quest Completion Dialogue
"I am not interested in who our opponent is. This was nothing more than a chance to quench my thirst..."
Given to you by a Jeritza near the entrance of the Training Grounds.
- Agarthium x1
- Reaper Knights
- 700 Renown
Subjective: The following part of this article is based upon the editor's personal experiences and opinions, and therefore may not be applicable for all readers.
Talk to the Jeritza at the entrance to the training grounds and accept the quest on the player's next day off.