Willows character then starts to go off the rails – for two specific reasons. First: she starts to make use of darker magic (instead of just lighter/safer magic). Second: she starts to make use of too much magic (instead of accomplishing some tasks naturally). Initially, Willow has no choice but to play with darker magic, when she undertakes a “Resurrection Spell“, with the aim of bringing Buffy back from the mystical death that she suffered (after defeating Glory).
The spell involves Willow speaking several incantations, together with: using blood from a sacrifice (within a specific urn), enduring several tests (cuts, lumps, a snake emerging from her throat) and powerful red fires engulfing Willow (whilst she casts the spell). Willow eventually succeeds in resurrecting Buffy, although she is a “little drained” afterwards (and as such, is unable to cast any spells – for a while).