Well, it's like I said earlier in the thread:
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The Tesseract ... is essentially where time-transcending entities like Lavos end up when they die. Since Lavos transcends time, he doesn't just "no longer exist" in the future, but in other time periods as well... Hence his entire existence basically gets banished to the Tesseract, which is similar to the Darkness Beyond Time -- or maybe it IS the Darkness Beyond Time. Anyway, that's the idea.
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Now, "normal" entities that die conventionally (such as a human getting killed) aren't erased from the past, only the future. So their "future" and all possible future timelines that include them in it end up in the Tesseract/DBT. Since Lavos was killed, boom, he was in the DBT. Schala is there at the time as well and he ensnared her, but we honestly don't know how she got there. We have theories of course (temporal disturbances during the Ocean Palace disaster may have caused a number of time paradoxes that we couldn't see, and without knowing where to "put" Schala, the laws of time just kinda threw her aside into the "trash can" of time, the DBT), but none are entirely 100% solid. But I digress.