The puzzle lists the first two releases of a videogame series created by Toby Fox and a hypothetical third release. Examining the the videogame names should establish fairly quickly that these three series members use the same letter set (ADEELNRTU). Additional examiniation should eventually reveal that the third release, LENRUADET, can be formed by permuting the letters of DELTARUNE in the same way that DELTARUNE can be formed by permuting the letters of UNDERTALE: The first letter goes to the seventh position, the second goes to the eighth position, and so on.
Using this transformation, we can generate further members of the series: Nteaduler (4th), Ereuldnta (5th), Eatdnleru (6th), Turlenead (7th), Rdaneetul (8th), Alueterdn (9th), and Undertale (10th, coming full circle). The puzzle leaves blanks to be filled in with the 7th, 8th, 5th, and 6th series members respectively. There are Turlenead, Rdaneetul, Ereuldnta, and Eatdnleru; taking the first letters of each as indicated yields this puzzle's answer, TREE.