I asked someone that has the title MMXX, which includes the triumph of killing Savathuns Suplicant, when I asked what did he do he/she said they roam around Titan for a while until it showed up, apparently a Taken enemy. Looked around for a bit but no success. If anybody is willing to look around and update this post that would be amazing.
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That triumph is not available yet. Most likely tied to Eris last lore piece.