The phrase "a grave challenge" is correct and usable in written English.
It can be used to describe a serious or significant difficulty or obstacle that needs to be addressed.
Example: "The team faced a grave challenge when they discovered the project's budget had been drastically cut."
Alternatives: "a serious challenge" or "a significant challenge".
The government considers any outside criticism of its top leader a grave challenge.
For those people and organizations working in countries in need of significant conflict prevention, mediation, reconstruction and reconciliation, these unhealed memories can present a grave challenge.
The violence poses a grave challenge for Mr. Sharif, who was voted into power after the general elections and only recently took office.
For members of the majority party in the House, signing a discharge petition is a grave challenge to the leadership's authority.
His expenditure of great sums of money on this enterprise and his innovations in taxation and legislation eventually brought about a grave challenge to his rule.
That would pose a grave challenge to the government of President Bashar al-Assad, who has relied on security forces to crush an unprecedented popular revolt against more than four decades of iron-fisted rule by his family.
Russia's decision is therefore a direct and grave challenge to the international order.
I do think that E. E. Cummings gives Graves a serious challenge, though I know that many sound people dismiss Cummings's love poetry outright, as self-congratulatory and goopy.
That transformation cannot be entirely written off as a masterful spin job, nor entirely attributed to a presidential growth spurt following the grave challenge of the terror attacks.
South Korea's President-elect Park Geun-hye has spoken of a "grave" security challenge from North Korea, following her victory in Wednesday's election.
In nine months, the growing Zika outbreak in Florida will present a grave new challenge to the nation's health system — babies born to mothers infected with the virus that causes severe birth defects, many of which we have yet to discover.
