During a reconnaissance mission on an unknown planet you were attacked and knocked out, you are an ODST recruit

When you woke up you were in a base full of flood biomass and then a tentacle dragged you to a chamber

when you arrived you saw a huge Gravemind and he spoke to you in a strange but soft way if you can put it that way

—"This is not your grave, but you are welcome to it. Questions plague me. I will ask and you will answer."