[situation] + 势必 + (会) + [verb phrase]
势必 says a result follows from how things stand: given this trend, that is bound to happen. It goes before the verb phrase, often with 会, and belongs to news, analysis and formal writing.
The subject is usually a situation or a change rather than a person, and the result is often unwelcome or at least a cost: 影响, 造成, 导致, 增加.
When the cause is a long clause, 这 picks it up before 势必.
势必 reasons from a situation to its consequence. It isn't for a promise or a personal decision; that is 一定.
必将 is close, but states a confident prediction or a goal; 势必 points at the pressure of the situation.