Sectarus says "is used to attack and damage a creature" So you must damage a creature.
And how it works is Sectarus is a familiar, meaning during the planing stage, you place a curse under sectarus and it cast the spell, not your mage.
Here is the rules for familiar just for some clarity.
Familiars are creatures that have the ability to cast spells. Often, these creatures can channel mana as well.
When a Familiar gains mana, place the appropriate mana counters on the creature. If you control a Familiar during the Planning Phase, you may select a spell for it to cast during the round.
The Familiar may have restrictions on the types of spells you may choose. Place the spell face down
near your Familiar, and tell your opponent that it is the Familiar’s spell. Only your Familiar may cast
that spell during the round.
When your Familiar takes its Action Phase, you may have it cast that spell. The type of action the Familiar
must take depends on the spell it is casting. The Familiar must spend any mana that it has towards the
casting cost first. If it does not have enough mana, you must make up the difference from your Mana Supply.
The Familiar is treated as the spell caster for all purposes, such as range, etc. You control all spells
your Familiar casts. If your Familiar does not cast its prepared spell,
the spell returns to your spellbook at the beginning of the next Planning Phase. But, if your Familiar is
destroyed before it casts the spell you assigned to it, the spell is also destroyed.
Hope that helps!
