Although the Vanguard has been reunited at last, their mood is not as celebratory as Ghost hoped.
Ikora believes that Zavala's encounter with the Witness has left him emotionally compromised and requires some time alone with him to assess the situation. She and the Commander will meet your fireteam up ahead.
For now, a mountain erected from the memory of Old Russia stands between you and the Witness's monolith. Ascend.