Ple-Sen
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This time, we'll just continuously cast the rune forward, ignoring any potential casualties among the civilian population, and that's it.
1) Alessia doesn't have infinite mana, and it will only last for five casts. There are potions, but they are not cheap and only provide one bonus cast.
2) In both battles with the bandit no-names, the rune wouldn't have done anything at all – both battles ended before the spell's effect could take place.
3) In the battle with the slum boss, the rune, again, didn't have a significant impact on the course of the battle – it didn't deal any damage and only served as a scarecrow for Isabella, forcing her to use one (potentially two) of her actions to move. It's not useless, as it saved Veserod's life, but it's not something that decides battles on its own or allows you to clear encounters without engaging in combat.
4) I expect the sewers to have an architecture similar to the "Princess" base, i.e., many small rooms and narrow, branching tunnels. Does it need to be said that this doesn't encourage the use of such spells?