A contract is a binding agreement between two people.  In the retroverse,
contracts are the business of the Cultists, who offer salvation of the body
(via dark magicks) at the soul's peril.  

Contracts are only entered by willing parties, by either the Evil
Resurrection or Binding Contract spells.  Evil Resurrection requires you
explicitly accept the spell, and allows you to decide the exact strength of
the contract (help spell evil resurrection).  Binding Contract is
implicitly accepted by joining the Cultist's party, and gives a contract of
random strength (help spell binding contract).  

The strength of the contract determines how well it holds up when the
Cultist comes to collect (with one exception noted below).  Once the
contract is collected, it is considered paid in full, and cannot be
collected again.  

Collect options:  
HP - The Cultist steals some HP, healing him and hurting you.
SP - The Cultist steals some SP, energizing him and draining you.
EP - The Cultist steals some EP, restoring him and tiring you.
disease - The Cultist gives you any diseases he has.  
poison - The Cultist switches poison status with you.  
hunger - The Cultist switches hunger status with you.
scar - The Cultist switches scars with you (gains all of yours gives all of
his).
location - The Cultist switches physical locations with you.
status - The Cultist gains status with his patron.  Unlike other contracts,
this one gives more status for a weaker bargain (0% is worth 20% more
status than 100%).  Only contracts from Evil Resurrection can be offered
for status.  

Status is by far the most common invocation of a contract, because it gives
the Cultist fuel for spells and advancement--but it is by no means assured
or required.  The cultist may invoke the contract for any of the above
options, at any time.  Even if the results are inconvenient or fatal, it is
not a valid reason to retaliate against the cultist, because you willingly
entered the contract in the first place.