Seas =)
First of all I like the spell and everything works fine - so I would give 3/5 (without these ideas) or 4,5/5 (with these ideas) - because:
1) The upper part of the portal is green ( on the gras ) don't know if that is fix-able but if it is you should do this.
2) You should base this ability on an ability with and AOE Circle ( Like Rain of Fire )
3) I think it better if every 10 seconds 1 demon spawn then only 1 demon spawn after the end of the duration ( 10/20/30 )
4) Maybe you should add some locust summoner at the ends of the circle. As you see ( at the screen ) these ends seems to be cuted. There summoner can be based on acolyte with stand work gold - animation
Note: Everything I said is nothing against you, your idea or you spell. This are just some points, how I think to improve this spell.