<em>Foreboding</em> means predicting or announcing that something bad is about to happen. Taking the answers one by one, you can see how they relate to the same ominous idea:
a "strange blight" and "evil maladies" refer to a destructive condition which no-body can explain;
an "evil spell" means that it is interpreted as a magical curse;
the "shadow of death" is a metaphor meaning that death is coming to the crops or cattle (depending on the original text excerpt).
It had a negative impact on the Native Americans, because the settlers were taking over the Native's land and livelihoods. As more Europeans came, the Natives were forced further and further from their tribal lands.
