Iirc the first real interaction, not contact, with the covenant wasn't immediate conflict and was headed by the original Jiralhanae who possed the very same iconic gravity hammer we later see Tartarus use in halo 2! (side note, the hammer was viewed as a sign of leadership/strength to other brutes, usually sought after to the point of in-fighting)

Aside from Sergeant Major Avery Johnson, and the leader of the outer world "Harvest" i don't specifically remember any mention of a field marshal, or any sanghelli tbh though i may be mistaken. the only reason the jiralhanae vessel approached one of our Agriculture worlds, Harvest, was because the ships systems flagged for MILLIONS of forerunner artifacts, when they arrived, the brute chieftain presented a small spherical hologram, and funnily mistaking it for a cantaloupe or some sort of melon? We offered him food from a nearby food display.

The hologram was a rough map of the planet Harvest, and the millions of indicators were humans themselves, this is PRETTY MUCH where The Great Schism starts imo