* U.N. Security Council visits Goma in eastern Congo
* Two other armed groups pose as great a threat as M23 -U.N.
* Millions have died during decades of conflict in Congo
By Michelle Nichols
GOMA, Democratic Republic of Congo, Oct 6 (Reuters) - After an aggressive new U.N. brigade in eastern Democratic Republic of Congo scored a military victory over M23 rebels and forced them back to ...